jhopper408

I know that I've seen this before but I can't find it now. What is the best synthetic rear end differential oil?
thanks
John

zwede

Dunno about "best", but many of us have had good luck with Mobil1 75W-90.

jhopper408

Thanks Zwede. Thats exactly what I wanted to know. I think that I'll stick with the Mobile 1.
thanks
John.

HeadsU.P.

Not trying to start an argument here, you can do want you want its your car.
But being old school, I believe synthetics have there place. And I believe that synthetics belong in the 2000s models. Seems like when people try to run synthetics in old cast iron situations with old school gaskets, things go wrong. A year from now you maybe asking why the rear axle leaks and chatters from synthetics.

I will stick with what GM still recommends for 40+ yrs:
2 bottles of GM rear axle lube.
1 bottle of GM posi modifier.

jhopper408

Wow! That's why I was asking. I'm old school too. So what's the wait of the GM oil?

jhopper408

So is there a certain weight that I should use?
thanks
John.

HeadsU.P.

IDK. But I found the bottles on Summit Racing and even Amazon sells it. Watch for different part numbers on the bottle. Not sure whats up with that. Maybe somebody knows the difference on here.

I pre-mix the two bottle types into another bottle which is clear and has the nozzle tip on top. (similar to a ketchup squeeze bottle) Then fill the differential with the mix. That way you don't have to explain to the cops why you are doing figure 8s in a parking lot.

Add: According to Amazon the GM bottle is 80W/90 wt. $17 free shpg.
Summit is $10 + shpg.

BKarol

The expert on the forum says to use 85/145. Post #4 https://www.corvetteforum.com/forums...ery-black.html

I bought 2 bottles of Lucas 85/145 and a 7oz bottle of the additive.

jb78L-82

I have been using either Mobil 1 or Amsoil 75W-90, both full synthetics, since 1990 (28 years now) without any GM additive, changing it every 8-10 years and have had zero issues with the differentials operation, sound, no leaks, no chatter.... Nothing. IMO Dino gear oil with additive is a safe bet, but modern synthetics are a better bet.......

Why anyone would use "old school" lubricants today when superior synthetic lubricants are available is beyond my scientific education unless cost is an overriding concern.

jhopper408

Thanks everyone for the excellent information!

jdmick

I'm still running dino in my rear but will be switching to Mobil 1 synthetic after I get it rebuilt. Mobil 1 synthetic gear lube SHC 634 has saved me tens of thousands of dollars and a lot of aggravation in my business. I have around 80 different gearboxes running daily and failures are almost unheard of. Was not the case with dino!

Big2Bird

Now THAT sounds real world to me. May I ask, for apples to apples sake, what kind of gearboxes do you speak?

7T1vette

Synthetic lubes/oils belong wherever there are rubbing/rolling/sliding metal-to-metal surfaces--regardless of their 'vintage'.

But, believe what you will and do what you want....

jdmick

Primarily 90 degree running conveyors along with some augers. The conveyor boxes are all imported and way too expensive. You could easily feel the difference in heat with one running conventional vs. synthetic. That also helps keep the seals from blowing out which is obviously very important when a breather plugs up. Only 2 horsepower maximum so we're not dealing with the kind of power that's going to an automotive differential but then again these boxes aren't built like a Ford 9" either. Bronze gears in many cases.

I also run 8 two stage 7.5hp air compressors. Synthetics in those have eliminated heat related issues with the pumps as well. The chinese motors on the other hand...

HeadsU.P.

Just looked back at some old threads about Syn in rear axles. On 4-4-08 up popped your name. You stated and I quote: stick with the original lube, stay away from Syn.
Something changed your mind in ten yrs.

7T1vette

Yes, it has. DATA and results.