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MG MGB Technical - Engine Oil


Which engine oil do you use?
Brand, regular/synthetic, grade, spec if known, ambient conditions, would be great.

Thanks, Rich

Mobil 1
Full synthetic (API SL)
Cheshire ambient conditions
Use it in both MGs (ZT and GT)

Downside, it's expensive, but should be serviceable for 12mths or 6k miles at least.

So far, since 3k mile running in on 20W50 mineral oil, the last 400 miles have seen no increased consumption (if anything less). Discolouration is minimal compared to cheaper mineral/semi oils at the same period.

Running pressure hasn't changed from the 70-75psi.
Idle pressure still 50psi, but dropping to 35-40psi if the car has been driven hard - this is understandable as I have dropped from 50 to 40 grade and nothing to do with the 20W drop to 0W which will actually improve startup oil flow - I have noticed the oil pressure gets up much faster than it did on 20W50.


Martin ZT

Castrol GTX 20w-50, labeled API Service SL. Exceeds: API Service SL/SJ/SH.
In North Carolina the annual temperature varies from around 10*F to 95*F. Yesterday was 82*F here. More information about API classifications is at:

Clifton Gordon

I have just done an oil change using Duckhams 20w/50

The reason for choosing it was that I wanted an oil with a strong colour and my memory was that Duckhams was green. This is so I can tell if oil drips from the bell housing as engine or gearbox oil.

David Witham

Valvoline 10w40 and I use alot more than I should. Pressure shows 26lbs at idle of 600 rpm and 70lbs above 1000 rpm. I am ready to switch back to 20w50 or maybe staight 40. I use about a quart a week. I drive about 40 miles daily combined intown and highway in FL.

Castrol 20w50.

I also prefer the Castrol because of it's ability to reduce wear. Seems the commercials about its ability to perform better than other oils in high revving 4 cyl. engines has at least a grain of truth.

My engine currently has 119,000 miles on the last rebuild that occurred in 1980. It IS getting tired (needs a valve job) but I am still only losing a little less than a quart every 3K miles.

When I put a new cam in it in 1996, the compression varied between 145 and 156 psi and it had almost 100K on the rebuild at that time. When I opened it up I was very happy with wear (actually the lack of wear) I saw.

This has made me a believer in Castrol. That being said, Valvoline is reputed to be very good as well.
Richard Smith

I've used Castrol in all of my cars for a million years, and of course Castrol GTX 20w-50 in the MGB since I bought it. At an oil change a few years ago, I used Valvoline 20w-50 instead, and noticed I had slightly better oil pressure. So now I swap between the two - depends which one I can get on sale. In fact, I have a case of each in the garage right now.
Matt Kulka

An old mechanic I used to use, (a real old timer, retired now, wouldn't know a computer if he tripped over it, but boy did he understand cars), would never use anything but Oilzum. He said it doesn't break down. Well anyway it's something that's not easy to find. I use Castrol GTX 20w-50.

I cannot say enough about how good Castrol GTX 20W50 is!
However, am thinkin' about going to Amzoil.............

Safety Fast
Dwight McCullough

Dwight, we used Amzoil in four of our 12 police cars, 2002/2003 Crown Vics. We lost valve trains in two, (If I have time, I will look for photos and send you one) and changed back to regular oil before we had problems with those, we have a loud lifter in one but we are getting rid of it in July and will run it till it quits. We sent samples to them and the test results cover Amzoil very well, as you would expect. We never pushed the issue. I use Castrol 20w50 in my 65-B and also in the OD-Transmission, totally clean inside and good oil pressure.
Steve C.
Steve C

Used Catrol GTX for years. I remember it first being introduced and looked down my nose at it as a 'boy racers' oil, along with go-faster stripes and that peculiar go-faster yellow. Anyway started using it when it was 20W/50, then noticed it had changed to 15W/50 but that was fine, then a few years ago it changed to 15W/40 and I noticed a definite drop in hot idle oil pressure in the V8. Tried GTX Magnatec (for older engines), which didn't seem to make much difference, and in any case is very expensive, doubly so as they diddle you out of 0.5L into the bargain. So now I use Halfords 20W/50. Whilst the roadster didn't seem to indicate the pressure loss like the V8 I saw little point in buying a more expensive oil for that, so they both get the Halfords, and the Celica gets their 15W/40. I have always changed the MGs at 3k or 12 month intervals. I've felt for some time that the 6 months interval is really intended for cars with a very low daily mileage, hence probably not warming up sufficiently. Although my cars are now low *annual* mileage when they *are* driven it is usually for a minimum of 30 or 40 miles (to get the Sunday paper from the shop 1/2 mile down the road), and often for several hundred miles in two or three days, so I don't have a problem with the extended duration as opposed to the low mileage. I always use oils advertised 'for older engines'. Those advertised for 'injection' or 'modern' engines are intended for use where the clearances are much less than our engines were when new, let alone after many tens if not a couple of hundred thousand miles.

Hated Duckhams Q20W/50, it comes out like brown sludge, it is only green when new! Although it depends on how fast it leaks I suppose ... (neither of mine do).
Paul Hunt

I use Duckhams 20W50 classic. At £12 a bottle I'd rather change it every 3k miles rather than leave a more expensive oil in for twice as long. The opinions are usually varied from cheaper oils with frequents changes to expensive oil with longer changes (such as Martin, who know his oils). If your engine is old and worn then I'd use even cheaper oil! (but still change frequently). I used to use the 20W50 in blue bottles from Halfords (tends to be away from main oils) at about £5 for 5L. But I was burning a lot and knew the engine rebuild was coming.

70 BGT 1860 stage 2 engine
Manchester UK
I D Cameron

I'm using Chevron Delo 15-40 for diesel engines. It is also rated for gasoline engines. Because diesels don't have catalytic convertors that zinc additives can foul, only diesel oils still have zinc additives. Zinc is the best extreme pressure additive out there.
With a modified cam and heavy springs I'll take every advantage to protect that valve train. Amsoil with a little zinc additive and would probably have no valve train problems. You can't find the additives on retail shelves any more in California. There are "off road" racing oils sold that do include zinc additives.
Barry Parkinson

Approx add pack of GTX and M1

Boron 2 170
Molybdenum 36 77
Zinc 881 1134

Top quality minerals are as close as they have ever been to Synthetics and agree with Paul H's general summary of use, and comment on Magnatec as only approx 5% ester.

However Sythetics are in a league (Premiership) of their own, re cold flow, stable viscosity, keeping engine clean, despite longer OCIs and ability to deal with extreme heat and fuel dilution.

Viscosity is just the same with Synthetic as Mineral, except that synthetic uses few viscosity improvers and is therefore stable, and all oil is too thick at start up, but a 0W or 5W will be as close as you can get to operating viscosity when cold.

High performance oils contain highly polar Group V Synthetic oil, usually esters.

Current UK winter use M1 OW40.


I once heard (from a machine shop that specialized in VW aircooled motors) that Castol contains parafin. Anyone heard this? Maybe it used to be at that time (mid-late 80s).

In the fall, I put in 10W-40, which I run all winter -- my B is a daily driver, about 8 miles each way, and rarely has time to warm up fully. Due to the colder weather, it stays at about 35-40 PSI idle, 55-60 PSI driving. Then in the spring when I change it, I put in 20W-50, which kicks it up to 50-55 idle/65-75 driving. The temperature here goes from -10 to -5 C most of the winter, up to high 30's and often 40 in the summer -- I find that this is a good compromise and keeps my B running happily.

In all cases, I use whatever brand they have cheapest at Walmart. I simply don't buy that there's any difference between brands worth considering, at least when it comes to nonsynthetics.
Chris St. Pierre

Extract re oil pressure

In an earlier section I said that thicker oils are usually needed in racing situations but not necessarily. Remember that a major function of oil is to cool the inside of your engine. In ASTM D 4485 3.1.4: “Terminology: Engine oil- a liquid that reduces friction and wear between moving parts within an engine, and also serves as a coolant.” Since the oil with a viscosity of 10 cS at 212 F thins to a viscosity of 3 cS at 302 F we will get more flow. The pressure will go down some as well. This is OK as long as we have a minimum of pressure to move the oil.

This increased flow will result in increased cooling by the oil. This is a good thing. You would probably want more oil flow in these situations and you get it. The hotter oil thins and this increases flow. The higher flow works harder to separate the engine parts that are under very high stress. It all works out for the better. Higher revving engines need thinner oils. You do not necessarily need to go to a thicker oil while racing. Only experimentation will tell.

The best way to figure out what viscosity of oil you need is to drive the car in the conditions you will use. Then use the oil viscosity that gives you 10 PSI per 1,000 RPM under those circumstances. For some reason very few people are able to get this simple principal correct. I cannot explain further.

These same rules apply to engines of any age, loose or tight. Just because your engine is old does not mean it needs a thicker oil. It will need a thicker oil only if it is overly worn, whether new or old. Yet the same principals of 10 PSI per 1,000 RPM still apply. In all cases you need to try different weight oils and see what happens. Then choose the correct viscosity.

I am using 0W-20 in my Ferrari 575 Maranello right now. It has over 5,000 miles on the clock. There will be a day (my estimate is 50,000 miles) when I will have to go to a 0W-30. In the future I will have to increase the viscosity to a 0W-40, then a 0W-50, maybe. I will use whatever it takes to give me 75 PSI at 6,000 RPM during the lifetime of my engine. This formula works in all situations.

Some people have tried this and occasionally get a somewhat low oil pressure while at idle. This is fine. There is no stress on parts at idle, the smallest oil flow will do the trick. It is at higher RPM where more BHP is produced. This is where we need the flow. Remember that Ferrari uses 75 PSI at 6,000 RPM as the place to test your oil viscosity needs. If your oil gives this value under your driving conditions then your lubrication system has been maximized. Period.


Valvoline racing 50w. Run hard on a 100 deg day and oil psi is still 50. And now with the Supercharger, have not seen any problem even with 200 deg water temp.

G VanHorn

Anyone got any experience with the British Comma oils? It´s the 20w-50 that is easiest for me to find, but of course I wonder aboute the quality.


I used to use Comma semi-synthetic oil in an Fiat Diesel ( a pre common rail car) . It ran a lot better when I changed back to Castrol Mineral oil and I have not used Comma since .This result surprised me since out of the two oils the Comma claimed one grade higher in API ratings In it's defence I have used Comma in various petrol engined family cars over the years without problems.I run the B on branded semi synthetic but avoid the 0/5W end of the spectrum. I change it once a year, and the filter every other year as filters actually peak for efficiency 1/2 way through their life .
S Best

To be API qualified, an oil is extensively tested on current technology test bed engines. This can cost a huge amount of money for qualification - (some figures suggest in excess of $200mill to develop and test).

So the Comma may have just met the higher API spec, whereas the Castrol may have exceeded the lower spec by some margin, and owing to cost was never retested to confirm it met or exceeded the next higher spec introduced later.

Martin ZT

I'm in a temperate climate and use Delphi (Lockheed, B&B) mineral 20W/50 SG from a motor factor. Seems to be a common spec for many of us. The car doesn't do high mileage and gets frequent changes.

I went for it as this was the last spec for "traditional" engines in 89, before they started tweaking for cats and including friction modifiers.

(Off topic, I wouldn't use later than SG in the gearbox/O/D as SH and later can contain friction modifiers.)

kids1, Truckers used to thin their oil with parafin in very cold weather, maybe that's where the story comes from?

Tore, I used Comma for a while in my daily drivers and had no problems. It was bought out by a well known oil company a while back and retained as a trade/budget brand. I'd trust it to be what it says on the can but over here, at least, the 20W/50 is SE spec.

It's quite difficult finding decent 20W/50 as much of it is sold as budget/old engine and not graded. Even Halfords Classic at £12 has no grade on it. (Nor did Castrol GTX in the 80's). Unipart do 20W/50 SF.


Hi Rich I use Valvoline 20w/50 racing oil, its widely available in the UK, you could use a lighter weight of oil, but I think these old engine prefere 20w/50. Synthetic oil would be a waste of money and as stated elsewhere it get pretty thin when hot, its designed for modern engines where tolerences are much closer than 'B' series units. Don't put 20w/50 in your modern car, the consequences can be expensive!

Terry Drinkwater

The looser the bearings (assuming an equal oil pump output) the lower the pressure. To maintain the pressure thicker oil is needed.
Racing engines are often built with looser clearances to reduce friction. Historically they were also built with those clearances because it was necessary to use a hi viscosity oil to avoid lubrication failure under sustained hi rpm hi load conditions.
Modern oils, even in lo viscosity, form have very hi film strength and tighter bearing clearances are reliable even in hi performance engines.

A pragmatic test - one that all seem to apply - is to run an oil that will maintain a good oil pressure. Very few cars today run 60 psi oil pressure. More pressure is not needed to maintain the lubrication layer within the bearing I have yet to read anyone's report that he used 10 30 weight oil and had bearing failure with his MGB. Ford is now retroactively specifying a 5-20 oil even for its heavy duty gasoline engines.
I ran 5-30 for awhile in my engine and it worked fine.
Because the rear cam bearing was installed incorrectly I had almost no lube to the rockers and had wear problems there. After 3,000 miles I tore down the engine and re located the rear cam bearing. All looked well, except, of course, the un-lubricated rockers. Although I didn't need to, I also replaced the bottom end inserts, new cam, pistons etc.
Critics of thin oil specs, assert that manufacturers are sacrificing engine life for gas mileage.
The other argument is that MGB's aren't designed for thin oil. To me a clearance of 1.5 thou" in an MGB is the same as 1.5 thou" in a new car.
With a modified cam the concern is the EP (extreme pressure) lubrication characteristics of the oil. The removal of zinc (the prime ep lubricant)from modern car oil may matter more as the oil viscosity is reduced. I have not seen any tests or data supporting that supposition.

Barry Parkinson


Synthetic oil runs cooler so opposite way round.


Both AW and FM operate in Boundary lubrication mode. ZDDP, esters, organometallic compounds such as Moly and Antimony build and maintain strong boundary lubrication films under severe load conditions and heat. Older first generation AW additives (cheap oil may use these due to cost) had little FM capabilities.

AW/EP films are semiplastic deposits which are hard to shear off. Thus, under shearing conditions, their coefficient of friction is high. The exceptions are the Moly and Antimony. With FM the outer layers are sheared-off easily, allowing for low coefficient of friction. "The phenomena can be described as a deck of plastic coated playing cards lying on the table and sliding off the top card easily."

AW/EP films work by protecting the mating metal surfaces when the oil film is ruptured, this layer of AW/EP material protects the mating surfaces from catastrophic failure.

The preferred film is hydrodynamic film. This is followed by the friction-modified, followed by AW/EP. When high speeds or low loads are present, it is easy to maintain the hydrodynamic regime. When the speed falls or the load rises above a critical point, the hydrodynamic regime breaks down and it is desirable to be able to go into a friction modification mode of operation. If no friction modification has been provided, the system defaults to a AW/EP.
The base oil and additives work in Synergy to provide protective and therefore additives should be avoided

Thin Oil - the case for
Warning this is very long

F1 uses Straight 5 or 10 weight (Redline off shelf!)but high revs
WRC 5W40 Motul 300V
B Race 10W60 Millers Synthetic (fuel dilution!)

Porsche specify (retrospectively) Synthetic with HTHS of minimum 3.5 (M1 is 3.6 and tested at 3.9 and contains all above additives + Boron). With M1 the motorsport is 15W50 Castrol 10W60 but with higher esters the viscosity can be dropped down a peg or two.
Redline has very high Zinc and Moly.

I'm reluctant to drop below the Porsche Spec but with non Synthetic/ester oils the lower weights are more shear stable, so using a higher weight to start with will end up at the lower weight.

I doubt an old 20W50 was stable, so probably using a lower weight anyway. A 10W30 should not therefore be an issue but I prefer a 0W or 5W40 Synthetic.


I haven't made a study of this but I use any of the name brands I find on sale. The important thing is changing it 2-4 times a year and using the appropriate weight for the season. Cheap oil thats fresh is going to be better than any oil thats been in there a long time. I also like to occasionally drop the oil, then add one quart, let the car idle ten minutes and then do the full on change. A lot of oil is held in the filter, cooler, and lines and at the cost of alittle time And one Quart seems like a good idea. Anyway my 74 has 116000 miles and shows 70+ PSI ant normal speeds. Thats good enough for me. One question though...Is a filter good for say 6000 miles between changing if the oil is refreshed every 2-3000?
Bob Ekstrand


I'm sure Martin can add to your filter question.

Filters get clogged and may therefore filter better as they clog but also restrict flow. A cheap filter with poor filtration may not clog as quickly and may last longer. If you prefer flow change filter as well.


This thread was discussed between 06/04/2005 and 10/04/2005

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