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Originally posted by charlie12:

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Originally posted by tatortodd:

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Originally posted by c1steve:
Mobil 1 is not rated for some diesels, so I would count that out. I just would not trust that it would be heavy duty enough for the gear mechanisms. Which brand matters little, what is more important is that you change it occasionally.

If you have limited slip in either differential, you should stick to what the factory says.

Mobil 1 is for passenger cars and light duty trucks, and Mobil Delvac 1 is for heavy duty applications.

As a comparison, Mobil Delvac 1 75W-90 meets a longer list of industry specs than Amsoil Severe Gear 75W-90.

The OP should look in his owner's manual and see what spec is required (e.g. API GL-5/MT-1) and find a 75W-90 from a reputable company (Mobil Delvac 1, Amsoil Severe Gear, Valvoline SynPower, Castrol Syntec, etc)

It's not going in the engine

I believe c1steve just clumsily worded his post as diesels require beefier transmissions, differentials, etc. Regular Mobil 1 gear oil is not rated Category MT-1 which designates lubricants intended for non-synchronized manual transmissions used in buses and heavy-duty trucks.

Mobil Delvac 1 gear oil is rated MT-1 and the more stringent military standard MIL-PRF-2105E (aka standard SAE J2360).

CT
I sent you a response to your e-mail

ptruck

Thanks to all who responded. Settled on Amsoil 75w-90 severe gear for front and rear. Now.....for the transfer case. Do I use 3qts of 5W-30 synthetic as some have started doing (there's a lot of interesting reading on this) or stick with the 2 qts of ATF? What a thing this information on the interweb is....

Thanks,

CT

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Originally posted by ptruck:I'm partial to the AMSOIL 75-90 severe gear, because I'm an AMSOIL Dealer. If you like technical reading, AMSOIL did a white paper called "A Study of Automotive Gear Lubes" (G2457). They tested 14 different gear lubes in this study.

After reading this and another report they did on motor oils, I became a big fan of Amsoil too. They are the only ones I know of that have hired an independent testing lab to run standardized ASTM tests on theirs and others oils to prove their point. These are tests that the oil industry uses. In almost every category, Amsoil was the best or in the top two or three.
Royal Purple consistently sheared down to a lower viscosity. I see it used on all the gearhead shows but I would not use it.

I formulate axle, manual transmission, and transfer case lubricants for a profession. Feel free to contact me directly for an opinion.