Going to change all the driveline fluids in the next week or so, and want to make sure I use the right stuff.

The owner's manual states the rear diff takes 75w140, but is such a heavy weight oil necessary? I was thinking that 75w90 might be able to help squeeze a couple-tenths more MPG out of this pig. Also, which fluid should I use for the part-time transfer case?

Truck is an 08 4wd 4.7 6spd w/open 3.92 rear

 

anybody?

 

I just got done getting my trucks first LOF @ the dealer (first one's free... who am I to argue with that?). I asked them about what fluids come from the factory in teh diffs & the transfer case, as I'm planning on going all synthetic soon. He told me that, from the factory, the rear diff has full synthetic in it, but the front diff and the transfer case still have dead dinosaurs in them. He also said that they use 80W90 gear lube for everything there, unless it's a synthetic. Hearing that, I see no reason not to go with the 75W90. I would highly recommend changing over to a good synthetic, though, if you're going to go through the trouble of changing them all out anyways.

 

Thanks...picked up some Mobil 1 0w30 enhanced mpg since they were out of 0w20. Will be running synthetic 75w90 in f & r diffs and synthetic ATF in tranny and t-case. With 3k on the truck in 2 weeks, oil change indicator is already flashing and motor seems to be idling a bit rough. Will be going with 10k oil change intervals using the Mobil 1.