Guru's Help! GL-4 or GL-5?

[DoubleDingo]

I have searched but came up with nada, and I don't have much time to spend browsing here at work. 1981 C20, Semi-Floater if I recall because it doesn't have the hub sticking out with the bolts on the end. Hub is the wrong word, but all I could think of right now. I want to change the gear oil since it has been in there for 33 years.

Which weight and spec would you recommend for durability and better fuel economy. I was thinking 80w90 GL-5 would be good, but I want some knowledgeable input before I open up the diff. I only haul heavy stuff once in a great while and not for long distances.

Thanks!

Shane

 

[DoubleDingo]

Was hoping somebody would chime in with a little insight on the matter , even a smart ass remark or something.

But I decided to go with 75w90 synthetic. Considered 75w140 but the engine is a stock vortec 350 so the horsepower and torque aren't extreme & I don't tow anything, and if I did it wouldn't be anything heavy or for long distances. The stuff I haul with the truck is only a few miles from the home improvement stores or the bulkyard, so 75w90 should suffice. I will report back if I get any increase in mileage over the old worn out oil vs the new synthetic oil. Even 1 mpg would be nice.

 

[bucket]

Honestly I don't think it makes much difference what we put in the rears of our old trucks, as long as it lubricates. GL-4, GL-5, 75-90, 80-90, smashed bananas... whatever.

 

[DoubleDingo]

... smashed bananas...

Now that's what I'm talking about......lol...

 

[kleedus]

I just put 80w90 in my 2007 with a g80 locker

I saw a thing from gm saying axles with that locker sometimes chatter in turns due to contaminated oil. it said to drain and refill with 80-90.

I fill all my axles with 80-90 unless it is a posi that needs the friction modifier stuff