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MG MGB GT V8 Factory Originals Technical - Oil Viscosity

I only noticed a couple of changes ago that GTX in
the UK has changed from 15W/50 to 15W/40. Might be my imagination but the engine seemed to sound more rattly than usual when hot with it. Decided to see if there were any other 15W/50s in Halfords and found two - their own brand and Duckhams Q. Couldn't quite bring myself to go for the own-brand so got the Duckhams. The first thing I noticed was that it seemed significantly thicker out of the can than I remember the GTX. 2nd was that the cheapskates only put 4.5 litres in instead of 5. But the bad news is the time the oil pressure gauge takes to rise from cold now. OK, it is a factory V8 which are well know for having a slow gauge, and we have ambients of 30/40F at the moment, but this is ridiculous. The only good news is that the hot idle is noticeably higher, but as I say it is winter.

Even though this oil has only just gone in I'm thinking of changing it again, but what for? Any better-flowing 15W/50s out there? If I have to settle for a /40 then maybe I should go for Shell Helix Super at 10W/40. Or even Mobil 1 at 5W/40 (although it also seems to be available as 0W/40 and 5W/30). My daughter's Metro was supposed to use 10W/40 as I remember, but it had the consistency of water and leaked past oil seals, whereas GTX didn't. Can I expect similar, or worse, leaks with the Shell and Mobil?

While searching for info I came across the following:

More Than You Ever Wanted to Know About Motor Oil
by Ed Hackett

The site has another link to a page on Octane.

Paul Hunt

I once used a well known synthetic type oil (can't remember which make)in my v8 and it made the engine sound like an old worn out B-series lump!especially with hi-rpm bleed down(rhodes)lifters..I once spoke to a guy from Shell(there is a a Shell research centre just up the road here at Pangbourn)and he obvoiusly recomeneded shell brands, but basically said that you should only ever use the grades the engine was originally designed for.So i think that 5-40 would be too far away from the original specs....If the oil guage is slow to register could a stiking oil pressure releif valve be the cause??

Paul, I too have got Castrol 'new' GTX 15/40 in my 1800, and I reckon it has more valve rocker noise than normal. It's also burning more oil than previously. I found some Duckhams Q20/50 in the original metal can at a Safeway filling station and will try that next oil change. As for the slow pressure rise on start up, I thought that was due to the 1800 filter being upside down, but the V8 has the can the right way up doesn't it? On mine, the pressure rises quicker with a Unipart filter than with a Champion.

Mike Howlett


I use Valvoline 20/50 but have noticed slow warm up at this time of year. I've been thinking about changing to Mobil and they recommend 15/50 for V8, Mobil should also flow more easily at lower temps.


I believe the Hardcastle book recommends using single weight 40 oil, and recommends against using multigrade oil. The guy that built my motor also recommended a heavy duty 40 weight oil. If my memory is right the book refers to the oiling system as a high volume low pressure system which works better with heavier viscosity single weight oil.

Mark Marchbanks

The reference to the single grade oil is actually for competition engines, however the principle of flow being primary over pressure is true for all engines.

For oil spec on the road engines there should be no reason to run outside the original specs which provide 4 options 10W/50, 10W/40, 20W/40 and 20W/50. It is interesting to note that with the introduction of the RV8 the specs were slightly different to include 5W/40 and 10W/40.

For the GTV8 the common spec was 20W/50 and the RV8 it is 10W/40. This is probably more a reflection of the change in general product on the oil market rather than any specific change in the engines.

However there is mute point which surrounds the fact that just like the B series engined MGB the V8 version is the only application using a standard oil cooler. The RV8 doesn't use one and it begs the same question asked of the 4 cylinder MGB as to whether it is actually a productive addition or not. I do believe that the ideal, just as with the 4 cyl cars, is to have an oil stat to direct oil through the cooler only when required. Incidentally this is a recomendation by David Hardcastle as well.

Roger Parker

Interesting comment re the oil stat. May be worth pursuing. Where can I get one, and where the hell will it fit in an engine bay where space is already tight bearing in mind it will (I assume) have to be spliced into the existing cooler pipes?
David Pollard

Oil stats are widely found at most decent speed shops, eg the Demon Tweeks type of operation (there are many others). One specialist that comes to mind is Think Automotive of Hounslow/ Isleworth area. They are usually H-shaped and are best fitted where the feed and return hoses run alongside each other, perhaps just before they pass through the front panel (can't picture the BV8 - I'm thinking of midget but the principle is the same).

I ran an original factory V8 on 20/50 close to the quarter million miles - tried 10/40 a couple of times but reverted to 20/50 - the engine still ran quietly when I changed it for a 3.9 three years back. It was also very clean internally.

The secret (that the motor industry doesn't want you to know) is that all oils loose a significant part of their viscosity at the 3 to 4K stage - The advice therefore is to change frequently and to use the cheapest available in the recommended grade with the highest cleaning factor - 10/40 - SG in europe and parts of N. America which have a proper winter or 20/50 elsewhere.

Comma, Morris or Halfords will do nicely (the scientists say) for ordinary motoring.

Happy New Year


Yes - it had/has still, an oil cooler without a thermostat

The local Halfods is selling Mobil 15/50 at 28 and Castrol GTX at 12. On the basis that Synthetics give improved mpg, only a marginal increase in mpg should save money over GTX enen keeping to 5k oil change, and also give the engine the benefits of a synthetic oil. Proof of pudding time.

If not running with oil thermostat may be worth adding a cover to cooler to keep oil temp up on cold days.


Re the oil cooler - I built a 4.0 litre MGB V8 ( standard rad and RV8 style exhaust) last year and, as an experiment, fitted no oil cooler.

It ran on the Prix des Alpes, driven quite hard up and down the Alpes and across France and the engine temperature never budged from a steady 190 degrees F - even in ambient temps approaching 100 degrees.

Not conclusive, I know, but I am inclined to think that the oil cooler is superfluous. I wonder whether the factory fitted it as "Belt & braces" in view of the sheilding of the sump by the front Xmember, or whether it just sounded more "sporty" to have one!!

I shall be fitting an oil temp guage to my own V8 to do some further experimentation.
Chris Betson

The comment about the cooler seems to remind me of a quote that it was originally fitted to boost the hot idle oil pressure, possibly on a Midget. Another is that it wasn't until the V8 was fitted to the MGB that the factory realised just how low the hot idle pressure can be, and also that the gauge plumbing was changed in an attempt to counteract the slow rise. Both good reasons for not having gauges, but warning lights and alarms.

Paul Hunt

I can vouch for what Paul says about the Midget. On my 1275, fitting a cooler made a world of difference to the hot idle oil pressure. I have a thermostatic valve on the oil cooler on my 1800 B. Here in Scotland it opens the pipes to the cooler radiator about twice a year - no kidding. I reckon in our temperate climate the cooler is unnecessary on the B. As I mentioned on another thread, I'm planning a V8 conversion at the moment and I'm seriously tempted to leave the cooler out.

Mike Howlett

This thread was discussed between 24/12/2001 and 28/12/2001

MG MGB GT V8 Factory Originals Technical index

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