Ryan77

I'll be draining and refilling all the fluids in my car this winter, and was wondering what's best oil to use for the rearend and manaul transmission? I heard Mobil 1 synthetic 75w-90, but I've heard the synthetic will probably start to leak. Also should I put in some posi-additive in the rearend? So what do you guys recommend?

Larry B.

You MUST.. absolutely must add the G/M positraction additive to the differential. I use valvoline limited slip gear oil 75/90 for both I tried heavier (140) in the trans but I could not shift it into 2nd when it was cold (by florida standards that's only 45F*) I went back to 75/90 and no problem.

nastee383

I don't know what the best is but I can tell you ROYAL PURPLE is pretty good stuff. I've had it in the rear, the tranny and the motor for years.

Eddie 70

I used the Mobil 1 in the tranny and rear end on mine. The transmission really shifts nicer now than it did before. Could be because of the new oil, I don`t know but I sure am pleased with it.

Ryan77

Silvervetteman-- do you use the Valvoline Durablend sythetic? that's all I saw for valvoline at Autozone.

Eddie 96-- Do you have any leaks with the Mobil 1 synthetic?

Thanks for all the replies :cheers:

Eddie 70

Ryan, my car has been off of the road for about a 10 months since all of this was done and I am now only to about 100 miles. So if it is leaking it is only a small amount and han`t hit the garage floor yet. Sorry I couldn`t help anymore than this.


[Modified by Eddie 96, 6:50 PM 12/26/2002]

Larry B.

No synthetic... I used standard valvoline stuff. As a matter of fact there was a discussion about the possibility of sythetic being too slippery for the posi-clutches. The posi needs the exact right amount of friction modifiers to eliminate clutch chatter but not slip under load. If in doubt use the GM oil and additive.

Wrecked82

I am going to put in my .02 for Lucas Oil Products. You do not, repeat, do not need to use the additive. I am sure that is going to cause some comment OK), but they say you don't need to, and 30K miles of use tell me the same.

Twin_Turbo

the additive is there to allow some slippage of the clutches, if you want to turn your posi shaft into this:



go ahead.. don't use the additive.

Marck

Ryan77

Don't worry I'll definately be using the posi additive. I'll probably get the regular valvoline, since I'm kind of partial to them anyway, I've always liked the products, I use their maxlife engine oil, coolant, power steering fluid, and auto tranny fluid (that one's for my winter car ;) ). Is 75w-90 the best to use?

Wrecked82

OK, Marck, I'll pull off some and put the additive in.

Larry B.

Several years ago I bought a 71 that had Posi clutch chatter so bad that the car was almost undrivable when it got warm. Starting up from a light it felt like the whole differential was comming apart. New oil and additive completely solved it in less than two blocks of driving. BTW .. you guys do know that '80 and up have a different differential. They may or may not react the same.. I have no experience with the 80-82's


[Modified by silvervetteman, 6:04 AM 12/27/2002]

Ryan77

I went to a few local auto parts stores today, and found out that 80w-90 is the recommended gear oil for the both the manual trans and the rearend for '77's. I bought the Castrol 80w-90, as it was the only 80w-90 gear oil that I could find in non-synthetic, and I also got tube of the posi additive. I'll probably do all the fluid changes next week.
Also, have any of you guys seen the article in the latest Corvette Fever magazine, about the drain plug kit for the differential's in C3's? It's basically a kit that gives you the parts/instructions needed to add a drain plug to the differential. Has anyone installed one of these?



[Modified by Ryan77, 8:02 PM 12/27/2002]