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MG Midget and Sprite Technical - diff oil

I finally got around to putting Mobil Mobilube 1 SHC 75w-90 into my diff and was very pleased with the results

after a 50+ mile run on the M1 at 70mph with the hood up the (reduced) diff whine I previously had was gone, I could still hear the diff but not as a whine

and whether itís because I filled the diff with less oil or the Mobilube is better seal I donít know but Iíve no oil on the outside casing that Iíve previously had

someone, sorry I forget who, on the BBS put that they change their (mineral?) diff oil every year and found that the diff whine they originally had got less until it disappeared

Iíd already changed the diff oil 5 times in four and a half years, the last time using synthetic diff oil (Castrol Syntrax Longlife 75W-90) two years ago and whilst the whine might have reduced it was still noticeable until changing to Mobilube this month, I wished Iíd used it sooner and canít think why I didnít as I intended to
Nigel Atkins

Big service for me next weekend Nigel - so I'm interested to read this.
Standard Spridget diff?
And is this magical whispering oil available widely?
Anyone have longer term experience?
Steve Clark

I used to use until it got hard to find and I had to switch to a synthetic Castrol product. Next diff oil change I'll search harder (ebay) to get Mobil.
Daniel Stapleton

Are we talking the rear diff?

I only changed that once and that was when I got the car and rebuilt the rear axle with new bearings and gaskets... so 13 years ago, I used lucas high peformance gear oil, ive never had a whin, or leaking, perhaps, I should change it if you all are changing yours yearly

If only there was a book with this kind of info that told what to do and when to do it.


Prop and the Blackhole Midget


How many miles have you done in 13 years? ;o)
Dave O'Neill 2

<<Are we talking the rear diff??>>

Why? - which one are you thinking of?
Steve Clark

with your past experience,would you think this would be ok in the gearbox. i have a slight whine in 2nd and 3rd. due to driving the car about 5 miles when i first got it with no oil in gearbox. just your personal opinion of course.
regards bob.
bob taylor

Here the term diff has become generic.

It can mean, the rearend, the gearbox, the 4x4 transfer case... needless to say you have to be explicit at jiffy lube when changing transfer fluids ... cause those guys are great at faking incompetance

Prop and the Blackhole Midget

This Mobil oil is just a touch lighter than the Castrol say about 1.5% to 4% depending on the measurement you check. So that is not enough to notice the difference just going by the specs, not actually using it as Nigel has just done and he notices a difference but what about in 6 months time as diffs are a bit hard on oil.

Mobil call it a GL5 that is suitable for GL4 use in gearboxes. In older gearboxes you would tend to go for a GL4 but as we all know the handbook say multigrade for UK use.

They are both synthetic oils.

Eddie Cairns

In the 13 years... ive done close to 12,000, but its sat for long periods of time collecting dust

I do more binge driving then anything... I wont drive it for along time, then ill drive it alot in a short period of 400 miles in a day is not out of bounds, I packed on 300 this past weekend just doing errands

Ill easliy do 1000 miles in mid sept

Dont tell the dmv... with historic plates im only allowed 1500 miles a year.


Prop and the Blackhole Midget

Hi Steve,
yes standard diff on my car

as you've already seen Daniel recommends it as same as me used Castrol as you can pick that up easier off the shelf but Mobilube is widely available via postal delivery or possibly from what ever fuel stations that stock Mobil products

here's just one source, ebay make have provides that have lower prices but I've not looked so don't know for sure

the secret of any oil change, engine, g/box or diff, is to be thorough, get the existing oil as hot as possible for draining and leave as long as possible to drain to get as much old oil and muck and crud out as possible

if your diff is well crudded up or has oil from the last decade or more you might consider using some sort of flush/clean before putting the new oil in
Nigel Atkins

before fast Eddie does I'll put I'm not an expert, or anywhere near on anything including oil

I'd say for your car no don't use the Mobilube as it's a GL5 - I stopped recommended it in other g/boxes when I found out Mobil had (originally it was ok but then they changed their minds)

your gearbox would normally have engine oil in and need changing more frequently that if gear oil is used, as gearbox oil is rarely checked let alone changed this does mean there are lots of car with very old and worn oil in their gearboxes

most owners put of straight or ribbed gearboxes use mineral 20w50 engine oil but not everyone, the range can be stretched and type of oil, some do use gear oil happily (see chart below for overlap of SAE and gear oil numbers)

if your g/box has faults on any gears the oil wont repair them but selecting a good quality oil in the right range for your use might help if only a little

as does a thorough oil change as I detailed in my reply to Steve, again you may want to start with some sort of flush/clean before putting the new oil in

MGB, which also use engine oil in their g/boxes have g/box oil changes listed in the workshop manual every 2 years or 24,000 miles whichever is the soonest so that what I'd look to do on an A-series Spridget

I think regularly driving the car helps plus at least you'll know if the oil changed helped or made no difference and if the whine gets worse

personally, only for myself, as I have a Ford type-9 gearbox that takes GL4 gear oil I might try Mobilube again and as Guy put Comma GL5 gear oil in his for 40,000 miles with no noticeable trouble and as I suspect my g/box isn't the best in the world because I bought it from rogues
Nigel Atkins

I doubt the Mobilube will wear much in 6 months as it states use in -
. On-highway light- and heavy-duty trucks, buses, vans, and cars
. Off-highway industries including: construction, quarrying, and agriculture

but I will be giving it a real world test, the next 6 months takes us through autumn and winter when I continue to use my midget which sits outside 24/7, 365/6

also in September I'll be again driving over the Welsh mountains in a very spirited fashion doing about 1,100-1,300 miles in 4-6 days (depends when the beer money runs out) so that will also give it a good test

I'll report back on my findings, good or bad (as I don't mind admitting to mistakes) - I won't be able to report on more motorway miles or much with the hood up as I do my best to avoid both
Nigel Atkins

sorry I forgot the chart for your post, here it is -

Nigel Atkins

I remembered wrong I did overfill this time too, possibly not as much as I usually do but I did have to wait for the excess to very slowly run out of the filler hole and as usual lost patience and replaced the plug before the oil fully stopped coming out

I used the same drain plug and tightened as usual so I'm still surprised that with the Mobilube there are no weeps and the case is still dry after a few spirited runs
Nigel Atkins

sorry I missed your comment before but just seen it now whilst looking for something else

Mobil did previously put that Mubilube GL5 could be used in the Ford box that specified GL4 but latter changed their minds, like you I had an old spec sheet but I checked by emailing Mobil to ask last year or two ago (it's in the Archives) and at that time they did not recommend it's use in g/boxes that require GL4

yet of course I know the link I've put up again says recommended for GL4, by this I think they mean for diffs not g/boxes but I'm not an employee of Mobil so don't really know what they mean other than when they sent me their email reply
Nigel Atkins

Nigel, I think the email reply you refer to was actually from Q8. At the time you said that Mobil hadn't replied.

The archived thread you refer to is entitled, very succinctly by Guy one must add, "Type 9 oil".

It's such a good read, and contains such valuable information, that I've re-activated it again.

Here's an excerpt for those who can't be bothered to read the whole thread.

"as with the thinner engine oil I have to put my hands up and say based on all this evidence that I got it wrong by using and recommending a GL5 - lack of thorough research".

It's quite interesting to read how that "hands up" came about . ;)

Lawrence Slater

thanks for that but unusually it's not that I've remembered wrong this time

I did also later, after the other thread had finished, get a reply from Mobil, I can't remember the exact details now and I didn't keep the email but basically IIRC it was kept quite general but had not to to use Mobilube in the Ford gearbox
Nigel Atkins

sorry I've just noticed I didn't delete part of my last post to you, I realised after looking at the Mobilube link that that was were you got the GL4 bit from and not that you'd read the same out of date info sheet I did a couple of years back
Nigel Atkins

Nigel, you said "because I filled the diff with less oil "

How much are you filling it?
Jeremy Cogman

Hi Jeremy,

it would have been more accurate to have put that I overfilled by less

dry capacity of the rear axle is 0.99 litre (info from Driver's Handbook of course)

I buy 1 litre bottles

I of course don't expect a drain and refill to take 0.99 litre because of clinging residue despite getting the old oil as warm as possible and waiting a reasonable time for it to drain

this time though I had more difficulty than usual getting the bottle above the axle filler hole and keeping the bottle's pouring spout squeezed enough to stop the oil flowing out so I had more wastage - shorter spout on this bottle I think

and despite my frequent practice at refilling the diff it seems my technique has got worse instead of better
Nigel Atkins

Your welcome Nigel. I like to help out when I can. :).
Lawrence Slater

David Smith

So is "fill 'till it overflows" considered too full. How much more than a litre is this?
Jeremy Cogman

best not to fill until overflow because the overflow run and drips can go on for ages

how much new oil goes in depends on how much old oil you've left in there

I start with putting in around two-thirds of the stated capacity but it's not ease if you just use the oil bottle as I did, better to find a more controlled instrument to fill the axle than just the oil bottle
Nigel Atkins

Jeremy, fill til it just runs back at you is fine; if you use a short extension tube on the filler bottle (10 to 15cm will do) it makes filling a doddle.
David Smith

I used to do it the way David has put but now I know better - or my rear axle is different to others spits oil out - learning by experience

excellent idea to use a bit of extension tube on the bottle spout - if you have any that will fit I did not, I'm not one of those with a yard full of Spridget parts and a garage or shed full of all tools and accessories - of course I could have postponed the job until I obtained a bit of extension tube
Nigel Atkins

I think you're right - you should have postponed the job, Nigel.
Nick and Cherry Scoop

It's always best to have the right tools for the job before starting; and having once found the right piece of extension tube it is not beyond me to store it somewhere logical so it is ready for next time. One of course does not have to insert the oil in large splurges; once the litre bottle is down to below three-quarters, simply pump it in a little at a time. It is also possible to insert the little finger into the filler hole and feel how far down the level is.
David Smith

Thanks Nigel & David, good to know what I've been doing for the last 25 years is correct.

I actually have a massive syringe like thing I use for filling it as I always used to struggle to get the bottle in especially once I had a panhard rod.
Jeremy Cogman

I use an oil can with a flexible pipe, makes your thumb ache though
Andy Phillips (frankenfrog)

I have two plastic bottles that I bought in France during a hub oil seal failure weekend, and both have a built-in flexible tube that you can pull out.

I do find it's best to remain calm and employ some semblance of method, rather than dashing at it.
Nick and Cherry Scoop

I was going to transfer two thirds of the Mobilube to an empty Castrol bottle that I have some tubing for - but - I forgot and threw the empty Castrol bottle away
Nigel Atkins

Real men use an AK46

(Available cheaper elsewhere btw, I paid much less for mine).
Jeremy Cogman

sensible men just remove the drain and fill plugs
Nigel Atkins

"Real men use an AK46"


You'll never get 72 virgins in paradise with one of those. Takbir.
Lawrence Slater

To clarify, I still remove the drain and fill plugs, I just use the 'syringe' to fill with. Much easier than squeezing nearly empty plastic bottles, especially, as previously mentioned, when you have to work round a p-rod.
Jeremy Cogman

well interim report ...

I checked the back axle rear case cover and diff oil level today

there was a very slight amount of oil around the filler plug which makes sense as it was a nip away from being tightly done up and we've been up and down a good few steep mountain roads recently

for various reasons we only done 830 miles on our Welsh mountain roads trip this year but it does mean that a greater proportion of the mileage this year was on those roads

diff is still quiet but we did run with the roof down on the motorways and all others roads the whole time

for Eddie I'll report back in another 5 months time about if the whine returns of diff breaks apart (unless either happens sooner)

so far I'm very impressed with the Mobilube in the diff and wished I'd used it sooner as I intended
Nigel Atkins

Looking for something else I stumbled across this thread and note I said I'd report back after 6 months of running the Mobilube in the rear axle - but then life got in the way so I'll report, what I can remember, now.

Eddie was right the whine did return but very much diminished, I can't remember now when I first noticed it return. I only noticed it when the roof was up and as I put it was very much diminished.

I think I can say using the Mobilube did help and it wasn't just a case of fresh oil improving things although that probably did help so there might be something in what another poster put about changing mineral oil each year to (perhaps) get rid of diff whine.

The Mobilube I put in August 2014, it replaced Castrol Syntrax Longlife 75w90 that I put in April 2012, so after about 19 months, and 10k miles, on the Castrol oil.

I fully drained and refilled with Mobilube again this July (2016), I think the slight diff whine is still there but can't remember as I don't listen out for it and have possibly got used to it so don't notice.

* I don't want to reopen the can of worms with the following so this is just for Daniel's and LS's interest if they read this post *
* - one thing I noticed when I checked the Mobilube link was still working was that on the "Specs" tab it has they recommend it "for use in applications requiring: API GL-4", so either a typo/misprint/error/ incomplete list/who knows! or they've done another U-turn - but I don't want to open that can of worms again! *
Nigel Atkins

I still have the lucas high performance gear oil 75-90 that ive had for years...still quite amd doing its job

1 Paper

IIRC I got my car with a bit of a diff whine but it was difficult to hear over the very noisy Spridget gearbox and noise from carbs, exhaust, tappets, hood, etc., surprisingly no squeaks or rattles that I can recall though.
Nigel Atkins

>>The Mobilube I put in August 2014, it replaced Castrol Syntrax Longlife 75w90 that I put in April 2012, so after about 19 months, and 10k miles, on the Castrol oil.<<

My bad maths there, should be - ... so after about 28 months, and about 10k miles, on the Castrol oil.
Nigel Atkins

As I put in my Type 9 oil (II / update) thread when the weather improves and I feel like it, around spring or summer, from the rear axle Iíll drain out as much of the existing Mobilube 1 SHC 75W-90 as possible and refill with Millers CRX 75w90NT and report back my findings.

Previous refills of the Mobilube have each stopped the diff whine for a period and the refills combined have reduced the level of the whine overall. Whether the effect would be the same from just regularly changing any rear axle oil, or just by using better quality oil, or a permutation of bot,h I canít say, I can only put what I find from what Iíve done.§or=Motorsport

Nigel Atkins

Oil does thin with use therefore with the Mobile Lube being in there a few years it will be a bit thinner.

The main seller of bits for MX5's, MX5parts would you believe, recommend this oil for both diffs and gearboxes in MX5's and the cars with the 6 speed boxes can have a difficult gear change when the car is first started in cold weather.

A number of owners up here in Aberdeenshire have used this oil in their gearboxes and have considered the gear change to rowing through treacle.

Changing to Castrol Multivehicle GL4 improves the change which infers that the Mobile Lube is a thicker oil than the Castrol even though the specifications do not suggest much difference.

Just for information Mk1 Mx5's 89 to 97 are now starting to have diffs that require changing, not every one or even quite a few of them but a number of then. The cars prior to 94 only took 0.65 litres of oil with the later cars taking 1 litre of oil. I am now changing diff and gearbox oil on all cars at 30,000 miles.

Recent used oil analysis of an Automatic Subaru, the front diff takes three times as much oil as the rear diff. It was obvious from the oil analysis of both diffs the one with the smaller capacity as the wear metals were 4 times higher in parts per million than the larger capacity front diff.

Therefore it makes you wonder why the recent MK4 MX5, only produced since mid-15 have a diff capacity of only 0.65 litres and those using them in competition are now reporting very high diff oil temperatures.
Eddie Cairns

Oils is a subject almost as complicated as tyres so as we find paper figures don't tell the whole story. Oils and their additives do wear and deplete so it makes sense to use an oil that starts out and remains better for as long a possible.

A leaflet for Red Line gearbox oils shows "viscosity loss in transmission use" of various gearbox oil types at after 5k-miles.

I can imagine the higher diff oil temps in competition use especially with the lower capacity but in competition use I'd have thought all the oils are changed very frequently 9or should be?).

Diff oil temp isn't a worry on my road going Midget but at a gearbox oil change, where I'd driven the car to get the oil hot for changing, I was very surprised how hot things were as when I got to siphon off the last bits of g/box oil I could a smoke-vapor drifted out of the plug hole. Just goes to show how hot the g/box gets even on a road car. I guess heat is also transferred from the engine, perhaps even a bit from the exhaust, but the prop shaft more or less isolates the axle from the built up g/box heat.

I find with using the Castrol Syntrans Multivehicle 75W-90 in my Ford Type 9 g/box that the gear changes feel noticeably better in the cold weather than warm weather.

When I had our Mk2, 5-speed, MX-5 new at the end of the last millennium the 6-speed box had just been introduced on the 10th Anniversary Model, later when a friend who liked cars from the 1930s and 40s bought a MX-5 after a few of us had been telling him how good they were (apart from the English dealerships of course, the main problem with most makes) he got a 6-speed new and had no end of trouble with the g/box, and perhaps engine IIRC.
Nigel Atkins

I finally got around to putting Millers CRX 75w90 NT transmission oil and was even more pleased with the results than with the Mobil which I see here I put in less than two years ago.

The diff sounds even quieter still now (tested at 50-70 mph for about 10 miles with the roof up).

I emptied out the Mobil and also flushed through the remaining Mobil left in the botttle from the last refill, about 100ml or so combined with about 200ml of Millers.

As always I got the existing oil as warm as possible, by going for a drive on a hot day and emptying out as soon as I got back.

Obviously this is totally unscientific report and only based on my hearing and perception of my diff in my car and a change of oil may have made things quieter anyway (but I doubt by so much as using the Millers, or perhaps another untested oil).

I still have a noise but it's quiet enough now to bring up another thought/issue perhaps which I'll leave to a more relevant thread.
Nigel Atkins

This thread was discussed between 21/08/2014 and 26/05/2017

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.