Can I use 75W-90 diff oil?

[NMuzj100]

A mechanic, very good one, told me that sythetics were excellent for engines and some manual transmissions and transfer cases, but not so good for front or rear differentials ...

I'm with you. Mobil-1 site list a number of units with factory fill on engine but none with gear oil factory fill.

Some of the perception among some mechanics that synthetics are unsuitible for differentials and cause differential problems comes from the recent introduction of synthetic gear oils in GM light trucks. GM placed synth gear oil in the rear diffs but to save costs they stayed with a "dino" oil in the front as they were not used as much (part-time 4X4s). The diff vent tubes and fittings (plastic) for the front diffs were not compatible with the new synth oil (to save a dime) and when dealers put the new fluid in the fronts it destroyed the vent system. This resulted in a TSB telling dealers not to place synth gear oil in differentials not originally designed for them. GM has since been installing synth compatible plastics in all applications.

Both Ford and GM light trucks come from the factory with synthetic gear oils. All the OEMs are going to them for the longer service intervals and efficiency gains. (the gains may be small but they want everything they can get)

 

[Morbo]

NMuzj100
Good information, do you know what brand sythetic gear oil is being used as a factory fill by GM and Ford? Another post stated there were differences from brand to brand. If so are they significant? Also are there basic differences in the diffs of GM,Ford and Toyota?

 

[NMuzj100]

On BITOG I read the GM synth was made by Texaco. But I don't have any confimation on that.

The GM syth gear oil (sometimes called "grape juice" because of it's purple color) is $26/qt dealer list.

I have no info on the other OEMs.

It will be interesting to see if Toyota is putting a synthetic gear oil in the rear diff of the new Tundra and or if they will still be spec'ing SAE 90. The new diff is incorporating other new tech and may have an oil to match.

... the iForce 5.7 models step up to a 10.5-in. ring gear – the largest in the segment.

All Tundra differentials are made using new machining technology: “face hob” gear cutting for ring and pinion gears yields stronger tooth form and a greater engagement area for increased torque capacity and reduced gear noise.​

Amsoil Tech sent me an email today saying they plan to introduce a SAE 75w-110 gear oil in the near future.

 

[CrusrDug]

I wish I would have found this thread before yesterday. I spent a couple hours looking for sae 90 diff oil and ended up getting 80-90. Then buying a separate 75-90 for the t-case on the advise of the auto store. In 15K I'll go to a 75-90 synthetic all around to keep things simple. What a pain in the arse having two different spec'd weights especially one that is not readily available.

This was my first change at 30K. I did notice the t-case was thinner and darker than the diffs upon draining. Is it because the 75-90 is thinner to start and possibly is has also broken down faster becasue of shear in the t-case compared to the diffs? I don't know if those are the right terms.

 

[Jim_Chow]

I bought a 5 gal pail of Mobil 1 delvac 75w90 and use that for both of my 4x4's.