75/90 vs 75/140 rear gear oil

[DennisG01]

Thinking of changing the rear diff gear oil:

(1) Is it OK to switch to full synthetic like Royal Purple (is the GM stuff already synthetic)? There are about 90K miles on it.

(2) Are there any advantages/disadvantages to 75/140 over 75/90?

[Idle_Chatter]

Synthetic lube is always a better option - more film strength, better heat resisitance, better flow capability. I don't believe that GM uses synthetic lubes in the pre-2001 differentials, but it's always an option. I ran 75W90 syn in my 99 6.5TD Taho (with a Mag-Hytec cover) very successfully. I also ran one batch of 90W140, and although I didn't tow heavy or often (which is where the heavier lube is really required) I didn't notice any negatives and the lube stayed clear and clean, which tells me my gears liked it.

As I said, the heavy lube is for heavy pulling, and the possible negatives are increased drag, especially in very cold weather - but the positives are the benefits of synthetics and increased film strength and lubrication.

[Robyn]

Just a point to ponder

I run good old dino stinky 85/140 gear oil in the tandem drives on the big truck

540K now on that set of eaton DS461's (46000 pound rating)

I have switched the 18 spd over to the eaton synth oil when we changed the tranny at 490K

The cost was horrible and the only difference I see is about 100F cooler on the gauge.

The eaton shop tells me they think the synth is a waste of time as the gear boxes seem to last just about the same amount of miles either way.
I drag 105,500 lbs around all the time and the last time we did oil change on the diffs we sent an oil sample in and it came back A OK with no contamination or metal.

Just some points to think about.

Possibly on the lighter units the synth may help with longevity, I dont know.

I think that changing the lube every 50K or so is a plus if you are running in situations that could cause more contamination or in very hot weather.

I have never changed oil in the little trucks unless it had to get dumped for a repair.

The big truck gets the diffs dumped and refilled every 100K period.

Just my 2 cents worth

Robyn

[JohnC]

Just FYI, our '99 8.5' w. G80 came with synthetic.

[Shikaroka]

I'm getting ready to rebulid my rear differently, and swap out the gears.
Is it ok to fill it up with synthetic during the break-in? What's the best to use, right off the bat?

[Scooby]

I have always do diff breakin with a good brand of standard ??w/140 oil, then switch to synthetic after breakin. Pay CLOSE attention to the breakin procedure for the new gearset. THe gears can be ruined if loaded long and hard during the breakin period. You DO NOT want to get the oil real hot and cause the oil to break down and not provide protection for the rest of the breakin period. Short trips with cooldown time is important. Long gearset life will be the benefit. You will be amazed at all the shiney stuff that comes out when you change the oil after breakin. Be sure to get that oil out when recommended. I use the dino oil only because it is cheaper- you could use synthetic for the breakin if you want. It would be harder to have thermal failure with the synthetic. I just neever push a new gearset in the first place, and have never (knock knock knock) had diff failure, not even on my mud truck, which is now running 5.13 gears.