Originally posted by wxmn6:
... I live in NY where we can see sub zero temperature during the winter), then would it hurt to go with higher viscosity if you don't do alot of heavy towing? ....

Does the front transfer case also use 75w-90 too? If I am not mistaken, the front transfer case does not have factory synthetic, correct? If I want to go synthetic, is there something else that I need to do (other than draining) to properly clean the case so it would not get contaimated with mixed fluid? Thanks. [/QB]
wxmn6,

I’m not an expert on this subject but from what I’ve learned unless you are towing big loads regularly it’s best to stay with the GL-5 synthetic 75W-90. Using the heavier 75W-140 when not needed can lower your fuel efficiency.

And yes last I knew the front diff comes with 80W- 90 mineral oil but you can switch to a GL-5 synthetic 75W-90 (the 4WD transfer case uses DEXRON III). However, GM recommends you have the “white” vent cover on your front Diff for compatibility with the synthetic oil. See Feb03 GM Techlink article for more info.

It’s also not necessary to flush or clean the diffs (front or rear) when doing a routine fluid change to synthetic – the fluids are compatible. Just remove the magnetic plug, drain and refill. Also clean the plug ... the front diff plug on mine had quite a bit of “goo” on it. A small amount of metal filings is normal. (see pics)

Personally I changed both front and rear diffs to Mobil-1 75W-90 at 5kmiles. I also changed the transfer case and Allison to Mobil-1 ATF.

A search of the forum will also turn up a quite a bit of info on this subject.

Hope this helps… [img]smile.gif[/img]