What Diff fluid does everyone run?

[dkvasnicka]

So the weather finally permitted me to change the rear fluid today. I found nothing weird inside... The fluid was dark but that was probably because it washed some more dirt that had been there prior to the first change. There was a little bit of extremely fine metal powder on the magnet (normal, I think). I turned the wheels to have a look at the ring gear and there is no visible wear on it, nothing seemed loose or visibly wrong... Cleaned it and put Castrol Syntrax Limited Slip 75W-140 in it. It's a 140 oil but has some interesting properties even compared to some 90 oils... like lower pour point (-52C, premium synth Shell 90 gear oil is dead at -40) and lower density at 15C. The LS additive in it is enough, I didn't have to add anything, it works.
However I still hear a slight howl, mainly when accelerating moderately while going through a curve (sometimes). That leads me to the conclusion that it's either the clutch packs or side gears or the small pinions...? Dunno.

 

[pL2005]

I bought my 05 Envoy XL with 173000 on it last May. I didn't know the history so I just changed everything out (all fluids, spark plugs, thermostat, serpentine belt, cleaned throttle body, etc) last summer after I got it. I used Valvoline Synpower 75w-90 fluid in the diffs, and Valvoline multivehicle transfer case fluid in the transfer case. No problems so far.

 

[BrianF]

Use what ever synthetic is cheaper. Synthetics usually operate at a lower temperature and can extend the life of the seals, gear set and the change index. No need to go to some expensive brand as most will be more than adequate. Most fluids come with the friction modifier added. it will not hurt the units that don't need it.

Unless you are doing heavy towing, serious offroading there is no need to go up past a 90 weight oil. The added film thickness is just not needed.

My 06 Tb has run great on 75w90. Currently it has Amsoil in it. Not because I needed a tough fluid but because it was 15 bucks a quart as opposed to over 20 for Mobil, RP etc. In rural Canada things are harder to get and more money. If I ever change it again I found an even cheaper supply of fully PAO based gear oil.

 

[gmcman]

Since I have a Redline dealer near me I changed from Amsoil to the Redline lightweight shockproof gear oil. This fluid reduced the light whine I had in the SS rear on decel. Prob neglect from the previous owner but has done well so far.