Rear diff oil. 85w-90 vs 85w-140

[amccanoe]

Hi all.

84 Eagle here with 3.08 rear end.

I'm getting ready to pull the rear diff cover and take a look inside.

My car manual calls for 85w-90 or 85w-140. Is there any downside to using the heavier stuff?  Any upside even if I am not towing anything?

And another problem. I went to two different auto parts places and Walmart. All they had was 80W-90...NO 85W-90. Can I use 80w-90.

Thanks!

[TheBirdman]

Either one would work fine, it mostly depends on your climate. If you live in the desert, youd want to use the 85w-140, or if you get winter most of the year like me, youd want the 85w-90. 80w-90 would probably be close enough for eagle work, its not like these things are heavy duty haulers or crazy power sportscars.

IMO, the best way to think about oil weights is that the first number, the "85" in 85w-90 is the base weight, thats the weight it behaves like when its cold, and the second number, the "90" is the weight it acts like when it gets hot, as there are really long chain polymers in the oil that curl up when cold and straighten out when hot, to keep the oil from getting too runny when its hot.