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MG MGB Technical - 20w50 gearbox oil

It has been 30 years since I moved to Texas in my MGB. That is the same time I began using a proper oil in my transmission. Manual calls for Castrol 20w50 motor oil in the gearbox, same as engine. 20w50 motor oil is equivalent in viscosity cold/hot to a 75w90 or 80w90 gear oil. Motor oil has anti wear additives and proper gear oil has EP (extreme pressure) additives. Although a gearbox is not the same as a hypoid differential where the gears engage, it is similar. Motor oil gives wear protection to bearings, while EP Gear Oil gives protection to bearings, plus it provides protection to the gear teeth as they mesh.

GL4 gear oil gives a minimal amount of protection to modern hypoid gearsets, while GL5 gives a better EP protection to these modern gearsets, and also provides friction modifiers. The MGB gearbox is no modern hypoid gearset with high EP needs. The GL4 gear oil has low sulfer compared to the GL5, and the GL4 contains zero friction modifiers. The low sulfer makes GL4 brass friendly, while the high sulfer GL5 is not. Also the GL5 gear oils are designed for modern synchromesh gearboxes, thus the friction modifiers.

Try putting gear oil designed for synchromesh manual transmissions in your MGB gearbox and see what happens. I can tell you the synchro's will riot. Why on earth would MG tell people to use motor oil in a gearbox? Oh, because the viscosity is just fine, and it contains no friction modifiers to mess with the synchros. Well guess what? Chevron has been making a cutting edge gear oil of GL4/5 spec that contains only the low amount of sulfer, with the added EP demands of a GL5 rating made up by boron and calcium additives. Being a GL4 oil it contains no friction modifiers. It comes in gear oil #s 80W90 or 85W140. Been using it for over 10 years and a lot of miles. Before that I was using a Valvoline product, until they only offered it up with synchro friction modifiers that the MGB synchro sets hated. The Delo Gear ESI is not synthetic, so will not leak or foam, and it is truly yellow metal friendly, almost as friendly as Redline MTL75W80, which the MGB transmission synchro's also hates.

Why on earth would anyone still be using 20W50 in their transmission?
S Akai

"Why on earth would anyone still be using 20W50 in their transmission?"

Why not? What problems has it caused in the 40-50 years these cars have been on the road? To imply that BL )other models were the same) didn't know what they were doing all those years ago and the millions of cars they produced that use engine oil in the gearbox, is pretty insulting.

The MGB GT V8 does specify gear oil for the gearbox, presumably to handle the greater forces. Which just confirms to me that BL knew what they were about.

Well said Paul. I am pretty sure that the Hillman Hunter for one also used 20W50 in their gearbox.
john wright

Wouldn't it be good if these discussions could be settled by the person who made the decision?? I've always wondered why you would put engine oil, with its detergent etc, in a gearbox. I would have thought the clue was "gear" box.
Allan Reeling

Allan, it does work well as far as the synchro's go. It is just the right viscosity (about 80 weight gear oil)and no friction modifiers have been added to throw off the somewhat fancy old school balk ring synchro's. Porsche used similar up to the 80's and so did Datsun and in the Mini's, for which 20w50 Castrol was divined.

Of all things, MG used bronze rings in this synchro box, and all those MGB owners that put their acidic used oil in the gearbox, or high sulfur EP oil, have had to rebuild long ago. Not quite sure why Paul is insulted, he could try to elaborate. Is it someone challenging his knowledge of something, or is he bothered by the spectre of having to think? Sorry about that Paul, but just because something works does not mean it is ideal, same goes for all your friends doing it does not make it the best choice.

BTW, I never said the Gear ESI was the only "other" thing that would work, I said it did work, and has all the benefits of why gear oils are specially designed for gears, and not engine bearings. The one I mentioned is the superior Chevron version of a German Porsche Boron gear oil you can't find here, and it costs about $6/liter, and will last in the gearbox quite a few miles. You don't need Boron though, unless you want to keep your bronze in good shape, assuming the parts have not been replaced with steel. Just try and find a bronze safe GL5 or GL4 gear oil that works in the MGB synchro box. I have done all that, The Redline, Valvoline synthetic and several other's. This is a valuable piece of info Paul.

Thing to avoid like the plague in gear oils for the MGB is any "synchromesh" additives, or anything with the very corrosive (bronze) LSD additives. The LSD additives will have you draining it out long before any corrosion to yellow metals takes place in the gearbox, because you will be grinding gear teeth every time, and don't put it in the MGB differential either.

As Allen points out, nobody here is holding a gun to your hear to use anything in your car you don't want to, and also nobody is forcing you to think about it either. Some people have logical minds and are able to think forward and think well. Others follow the others and do simply what works, with no regard to longevity. Don't bother me what you want to do. Anybody want to share other lubes they have found work good in the gearbox I would enjoy hearing about it.
S Akai

Interesting, never thought that way on this topic S Akai. I have been using Royal Purple Max Gear 70-90 for many years now. When I saw a piece about how to translate engine oil weight to gear oil weight I was surprised it sounded so strange that motor oil like Castrol GTX is like gear oil in thickness when they both get hot or cold. The Max Purple has no limited slip modifier, maybe that is why the synchronizers are so happy and it shifts so effortlessly? I was noticing poor sync when downshifting 4th to third when all warned up. After switching to actual oil designed for gears, it stopped doing that, and has remained not doing it.
T Barrows

"Not quite sure why Paul is insulted"

Insulting to BL.

"same goes for all your friends doing it does not make it the best choice"

That's rich coming from someone who wants to do something different to what the factory recommended. 20W/50 may well not be appropriate for gearboxes in modern cars, but then neither is Mobil 0W/20 in our engines.

"Just try and find a bronze safe GL5 or GL4 gear oil that works in the MGB synchro box"

Like I said BL recommended GL4 gear oil for the MGB GT V8, mine is going round the clock for the third time on it. In the past a number of people have said that gear oil has made their 4-cylinder boxes very heavy in cold weather, even though it is much the same basic viscosity. I used my V8 in all weathers including parked outside and having to scrape the snow and ice off and did not experience that.

Anyone can put whatever they are most happy with in their gearboxes, some swear by ATF but that is usually because their OD experiences problems with conventional oil. Nowhere did I say you can't use gear oil - although you almost certainly would have problems with GL5 differential oil.

I'll remind you of the closing paragraph in your first post:

"Why on earth would anyone still be using 20W50 in their transmission?"

It's the implication that anyone who still uses what the factory recommended must be wrong, that I objected to.

The main problem with GL5 is it's too good! The additives "bond" a hard, sacrificial layer onto the metal. On soft, copper alloys, rather than scuff off, which they are supposed to do, friction strips them off together with the bonded layer of the base metal.
That is not good in MG difs either, as it has the same destructive effect on the thrust washers.
Allan Reeling

I've always used GL5 in my axles. The bonding affects yellow metals in gearboxes it is true, as the mechanical process does strip the lubricant off taking a microscopic layer with it. But as far as I'm aware that doesn't happen in our diffs. If it does, it's taking a blooming long time. And I may be putting my head in the lion's mouth again, but BL gave the higher spec for axles compared to GL4 in the V8 gearbox.

I bought a new 1966 MGB in May of 66, and the BMC dealer serviced the oil changes for 4 years, then I did my own service. The dealer used Veedol 30 W non-detergent oil in the gearbox; & the engine used 20 or 30W non-detergent depending on temperature/season. After 200K miles, the crankshaft remained std for last rebuild. The only gearbox issue was worn layshaft, needle bearings & synchro's. Presently have that engine & gearbox in my 67B.
Rich Boris

Thanks Paul, we are not talking about differentials.
We have got several different things that work in the gearbox now, being reported. The gears will last longer with a correct type gear oil that the synchro's like. Period. So what if you got 300,000 with Gemco recycled motor oil? Not talking OD gearboxes, either
S Akai

Mr Akai, I use EP90 oil in several older MG gearboxes, but have long been happy with 20W50 in the'73 BGT box. I'm happy with that and whilst, as you say, the MGB manual speaks of 20W50 in the gearbox, I'm also not particularly at odds with the statements in your first couple of paragraphs. Since the 1960s, I accept that oil chemistry has moved forward to keep pace with engine, transmission and diff. development, but I've never felt any need to put anything other than good basic "old school" oils in my engines and transmissions; and perhaps as a tradeoff, change the oils reasonably regularly.

Where I do object to your postings is that you feel a need to infer that anyone who doesn't take your view, is some kind of mug. The general idea and tone of these BBS forums is I suggest a frank but cordial exchange of views - including by all means polar opposites at times - but without any need for personal slurs of the sort you proceeded to direct at Paul Hunt. Statements such as "is someone challenging his knowledge of something, or is he bothered by the spectre of having to think..." - for goodness sake man, where did that invective come from, and why was it necessary?
For what it's worth (I can imagine your response!), as a keen amateur restorer I found your posts rambling and confusing and therefore hardly constructive. Paul Hunt, for my money, has long shared a wealth of information with fellow enthusiasts, without inferring ignorance on the part of those of us with lesser knowledge or who might beg to differ.

Could I humbly suggest that you keep your posts concise, relevant and respectful of the wider community.

With kind regards, John Hall.
J P Hall

Here is a British Leyland Technical Service Bulletin from 1975. SAE90 Hypoid is recommended.


I have been a follower and contributor to this forum for 10 years now and read this thread the day it was posted. I decided then and there I would not contribute because I could see that anyone else's opinion was unlikely to be appreciated.
I will just say thanks to John Hall for his summary which reflect my views exactly.
Ian Buckley

JP, your pretense of sincerity is not fooling anyone. Why don't you answer the question you find so offensive; rather than repeat it like the other's who also cannot answer the question, except to say that the 20w50 works fine? Yes it works fine, I have said so much. Something else might work better to make the transmission last longer. So answer the question. Why use 20w50 when a few gear oils will work fine with the synchro's and cause the gear faces to last longer? DO you know what a question is? What on earth is wrong with you that you think I presume to be GOD telling everybody what oil to use in their gearbox? How utterly offensive of you. The question was to create discussion. Is that really so hard to fathom? There, that rude enough and concise enough? And quite frankly, I don't give a rat's ass about your guru guy.
S Akai

The sensible thinking members of this forum should consider this item CLOSED and unworthy of further discussion, given the stance of Mr. Akai.
b row

I suspect that S Akai is visiting us from the MGE board, where he gets the boot every day or two, and then signs back in under another name.

C R Huff

Same people who like bias ply tires, it sounds like.
Because you "like" bias ply tires and motor oil in the gearbox is a good enough reason for doing it, but it does not explain why.

Charley has the MGB owner playbook down exactly. When something most all of them do is pointed as being nonsense, you just make wild claims about the person saying something they never said, say he is being rude and insulting, or kick them out of your treehouse. boo hoo
S Akai


So you think S Akai is really Prop?

AKA Woodpecker. RAY
rjm RAY

AKA Sam Coleman from Stonebridge, Kansas, AKA Nicky T, Duncan M, Roger Rabbit, Portree, Woodyredpecker, etc, etc. Just another deeply disturbed troll who will eventually get bored and move on to bedevil people elsewhere.
Stephen Strange


Roger rabbit... nice

Nicky T ....? Whatever

Cmon S Akai

How bout something daring like....

Tits Magee

After many years of participating on this board, this is the first time I've been convinced of the presence of a "troll".

It is obvious that this S. Akai has nothing else to do with his time, and that he derives some sort of perverse pleasure from being an irritant to "normal" (if you can consider people who own LBC's to be normal)people.

Trolls are always best left to "stew in their own juices", and when they discover that they can't irritate anyone, they simply move on to somewhere that they can "get their jollies".
JR Jim) Ross

SD Devine, how about instead of tits magee a name better suited like a slight change to some of the other phony names being used here on this board, such as:
CR Huff
Jim Ross
B Row
Sam Coleman

Anyone for a beer------
Ahh--another please-- this could take a while

William Revit

Have a Stella Artois for me Willie. Let this die!!!

79 MGB
gary hansen

Cheers Gary
William Revit


I just noticed this new one...pretty funny!

This thread was discussed between 23/08/2016 and 06/09/2016

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