75w-90 Synthetic Gear Oil - Best Gas Mileage?

[mechtech2]

A change to full synthetic anything can leak , even though there are no 'false seal' deposits.
It is not supposed to, but it happens.

 

[Panzerman]

Dodge calls for Rearend oil changes every 15,000 miles, with full synthetic 75w90 in thier 3/4 ton trucks, this is not only insane, its exspensive. Required to maintain your "Lifetime" warranty. The stuff looks like new when ever I drain it.

 

[qdeezie]

Originally Posted By: CBR.worm
Is 75w90 the correct viscosity? If not I would be worried about R&P wear towing heavy loads.


According to the service manuals I've bought for it, they all state 80w-90, so I decided to step up to a synthetic blend of 75w-90 as a test run and it seems to work ok. I did some towing before changing it and it towed fine, so I have minimal concerns there. If I was towing every day or once a week, then I would definitely step up to 75w-140. But a few times a year won't hurt anything.

 

[qdeezie]

Originally Posted By: mechtech2
gdeezie -
If you note a change with your new oil of the same viscosity, please let us know. I am thinking it could not be measured.



I will definitely let everyone know.

 

[qdeezie]

Originally Posted By: Doug Hillary
Hi,
qdeezie - The specification is important. I ran two similar 75W-90 synthetic lubricants in the Eaton diffs in my Class 8 trucks in Interstate work

The OCIs were 1mkms (620kmls) and UOAs were taken off all trucks at regular intervals

The Castrol and Mobil products performed almost identically. I had always thought that the Castrol product - which I already knew very well - was the better - they performed the same!

The move from a 85W-140 mineral to the 75W-90 synthetic reduced operating temps by between 15 and 20C and virtually eliminated oil seal replacement

The correct spec is always important - the advertising less so!


I'll be sure to keep that in mind! I also took a look at Castrol Syntec 75w-90 gear oil since it was relatively cheap.

 

[qdeezie]

Regarding seepage, it's a risk I'll have to take. If it seeps out, I will have a weekend seal replacement job ahead of me. I've been considering seal replacement anyway, so I might just do an all in one just to mitigate the risk. I'm thinking that the fresh fluid I'm using for 10,000 miles to clean it out should give me some sort of seepage indication prior to going full syn, but then again full syn is on a different level.