I need to change the oil in the rear diff. I was thinking about the synthetic 75-140. Is the synthetic oil worth the extra money for the rear end, or is regular dyno 80-90 ok? What do you think?

 

Mine calls for 80w90 but when I serviced it after I bought the truck, I upgraded it to 75w90 synthetic for peace of mind. I don't tow super heavy, so I'm sure it would live forever with the dino oil in it too.

 

If it were me I'd definitely use the synthetic. If you don't tow, you're better off with the 75w90.

 

80w90 for normal duty, 75w140 if you intend to pull heavy stuff. the D80 is a bullet proof axle either way

 

I use Royal Purple 75w140 in my differentials.
Figure it's worth the extra price since I only change the fluid every 100k.
If you don't tow 80w90 is fine.

 

75w90 synthetic is best all around for dd or towing, that's what I've used for years in semi's, medium duty's and pickups. Have one semi with 1,200,000 on rear def running 75w90 changed at 250,000 mile intervals, power divider failed in front def at 800,000 and put metal into the def so I swapped out the whole unit.

 

Thanks Infidel. Thanks guys. I ended up using valvaline 80-90 with the additive for Ls. Called the valvaline 800 # and that's what they recommended. I didn't have much time so I went with what they said. I appreciate they info. I'd be lost without this forum!