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Is Castrol 85w-90 gear oil correct replacement oil for front and rear differentials in m38a1/M170 axles?

Virtually any modern Hypoid type gear oil is acceptable for use in your axles. The choice of weight/grade depends on your seasonal weather and your personal likes. The word "Correct" is difficult to apply to our restorations especially when you don't tell us what correct means to you or what the intended use of your jeep is.

hello wes - its a factory restoration process on a m170. Live in North Carolina. Antique Car shows first & then putz around after vehicle has been shown a few times.. Would like to use correct specification and thought the 85w-90 gear oil would be a good choice... yes - no

If you keep telling me "Correct Specification" then the answer is no. The "Original Correct Specification" was GO 90 year round and was later updated to GO 90 summer and GO 75 winter. Multigrade were not in the M38A1 specs.
Unless you truly believe a judge at a car show is going to take points off your score for GO 85W-90 when the original spec calls for 90 or 75 straight weight I would use the modern year round 85W-90.

Agree..I just wanted to make sure i was not putting in an oil that would not provide the correct lubrication..not worried about judge, more concerned about providing correct lubrication.

Thanks for your feedback. I was also checking out some Lucas products...


Thanks again....

Keep in mind that the vast majority of these half century old jeeps have their original differentials still in them and they just spent a half century with old fashion straight weight gear oil in them. If they just get a good clean out, new bearings and another 3 pints of good ole fashion straight weight 90 gear oil they will most certainly last another half century.

The moral of this short tale is all these hyper duty modern lubricants are designed for modern cars, very high operating temps and and much higher speeds then our jeeps will ever be capable of.

Hello, this is my first time posting so I hope I'm doing it in the right way. I have an M38cdn and I'm hoping some one can tell me exactly what to use for diff fluid and where to buy it in Ontario Canada. The only thing I've been able to find at Canadian Tire or Part source is GL5 and from what I understand I should not use that in my diffs. If anybody could tell me of a specific product that would be so greatly appreciated. Thank you!!!

Would this product be ok?

http://www.canadiantire.ca/en/pdp/motom ... p.html#srp

Thanks!

I wouldn't say you cannot use GL5 in your axle gears. Above discussion means that the use of modern axle lubricants is overkill in our half century old 45-50 MPH jeeps. If the modern fluid is all you have available then by all means feel free to use it.

However I strongly advise against using the GL5 in your transmission!

Thank you for your reply!!

The gear oil from Canadian Tire will work fine in your diffs. The concern with that fluid is when it comes to using it in your transmission and transfer case. Because it is gl5 "hypoid" oil it contains sulphur designed to form a barrier on the gear faces and reduce wear. That works very well on diffs but when it comes to the soft brass parts in tranny and t-case that additive peels off microns of brass as it forms the barrier and wears off over and over.

That's not to say that it won't work, or that others haven't been using it successfully for a long time, it just leads to premature wear when compared to GL 1-4 rated oils. However, good luck finding GL 1-4 oils, or even just gear oil listed as safe for brass up here - I'm still looking.

I found a guy in Burlington Ontario selling GL-1.
Havwnt picked it up yet but hope to in the next few weeks

southpw wrote:I found a guy in Burlington Ontario selling GL-1.
Havwnt picked it up yet but hope to in the next few weeks

Is that the place that imports from England?

I really dobt know Jordan but I assume it is since it is Millers oil and lubricant that he is selling.

Sounds like a great lead! Please let me know how it turns out.