So what's everyones preference on rear differential gear oil? I just picked up some Mobil 1 75w-140 synthetic. Had a hard time finding anything that wasn't either store brand name, or seemed everywhere had Royal Purple which I almost got.

 

mobile 1 is better IMO I have that in my diff with maby 2k since the change.

 

I'm an Amsoil Severe Gear 75w-90 kinda guy.

 

I here that it supposed to be good oil as well....some swear by it, but isn't the rear supposed to have 75w-140? I bought my truck in 1994 brand new. Shortly after I changed mine and I remember asking what I needed, was told "just good ol gear oil" pretty sure I put in 75w-90 NON-synthetic Valvoline. Hasn't been changed since. 17 years and over 250,000 miles on that oil without any sign of a problem...but think it's time for a change. So NEEDING synthetic....not true.

 

According to my research, our trucks came with 80w-90 oil. From experience, if I was doing extremely hard driving or constant towing, I would use conventional 80w-140, 85w-140 or synthetic / semi synthetic 75w-140. For regular driving or for occasional towing, 80w-90 conventional or 75-90 synthetic / semi synthetic. In my honest opinion a lube by a reputable manufacturer is the absolute requirement here. So, valvoline, castrol, redline, amsoil or royal purple is ok. I would only use a cheap off brand lube as a 5k-10k flush prior to going with a good quality lube. I eyeballed castrol syntec 75w-90 pretty hard because it had a decent price tag. If one wants to err on the extreme side of caution, the 140 weights are the ticket.