75w-90 Gear oil in Rear diff

[JBakerFX4]

I know our rear diffs (sing rear wheel w/sterling axle) call for a LS 75w-140 Syn GL-5 Gear Lube. Well it crossed my mind lately since I don't tow or even haul anything over a few hundred pounds I might gain some MPG's by going with a LS 75w-90 Syn GL-5. And to help keep the lube alive, and from thinning out I'm going to do the 2008 Finned Diff cover. That will help the thinner oil stay cooler and in better shape should temps get out of hand. what do you guys think? I just did a 1,000 mile road trip with 80w-90 Syn Blend Valvoline and it ran great. I understand why they call for the 140 though... the truck is rated to pull 24,900 #'s and it's a 3/4 ton HD truck. But I just don't use the truck even close to what it's desinged for.

[Dirk]

The 75-140 syn will flow the same as the 75-90 syn at normal axle temps.


For piece of mind, I will always run the 75-140 and not worry about whatever I'm hauling or pulling.

[JBakerFX4]

10-4. with my luck I'll end up towing something right after the change. SO... going to stick with the 75-140. Not sure if I should go with Mobil 1, or Valvoline Syn.

[HeavyAssault]

Royal Purple makes a 75-140 and doesn't require any of the LS additative. I've had my
'08 cover w/RP fluid on for quite a while now with no issues.

[lightninmike]

Originally Posted by HeavyAssault

Royal Purple makes a 75-140 and doesn't require any of the LS additative. I've had my
'08 cover w/RP fluid on for quite a while now with no issues.

I would still still put some additive in. I just swapped the fluid in my F150 with the RP and i had to add some LS additive as i was peg legging. Sucks to have almost 600hp and you one wheel peal

[67ModStang]

So then why is it that the DRW trucks with the Dana 80 call for 75w90??? besides that it holds about a quart more oil.

[cartmanea]

It was my understanding that the additive makes it slip easier.

Originally Posted by lightninmike

I would still still put some additive in. I just swapped the fluid in my F150 with the RP and i had to add some LS additive as i was peg legging. Sucks to have almost 600hp and you one wheel peal

[CoalRollinRoosterCruiser]

Originally Posted by lightninmike

I would still still put some additive in. I just swapped the fluid in my F150 with the RP and i had to add some LS additive as i was peg legging. Sucks to have almost 600hp and you one wheel peal

That's a bad idea. That will leave you with wayyy to much modifier.

If anything, the fluid's that already have the modifier in it, have too much.

[Dirk]

Originally Posted by HeavyAssault

Royal Purple makes a 75-140 and doesn't require any of the LS additative. I've had my
'08 cover w/RP fluid on for quite a while now with no issues.

I run the amsoil 75-140 and the Slip-Lock Differential Additive.


On corners, it will break loose and won't hardly lock back up going straight.

[JBakerFX4]

Good question why the Dana 80 DRW take 75-90, but the SRW Sterling 10" take 75-140. I don't think we'll ever really know...