rear differential fluid

[Nigel]

Hi there. My mechanic is going to change the rear diff gasket next week and I wanted to check what grade differential fluid was correct for my car. It is a 1968 RS / SS 350 L48 with it's original rear end.  If there is a particular brand then that would be good to know too. Also the quantity I need to purchase.

Many thanks in advance, Nigel

[WorkinProgress]

Is it a posi rear?

           - Warren

[Nigel]

Hi Warren, To be honest I am not sure. Is there any markings on the rear end which will tell me one way or another. Many thanks Nigel

[69Z28-RS]

over the many years since these cars were built, many rears have had their internal guts replaced, thus the factory markings don't tell yuo reliably what is inside (whether Posi or not).

To know if it's a posi, jack up the rear (so both wheels are clear of the ground), place the transmission in Neutral.   Then turn one of the rear wheels.  If the alternate wheel rotates in the same direction, you have a posi.  If the opposite side wheel rotates in the opposite direction from the one you turn, then it's not a Posi.

If it's a posi, then you need to add a Positraction modifier to the rear end dope...

[Nigel]

Thanks for the info. Is there a particular grade of rear end fluid I should buy? Mnay thanks Nigel

[Randy Shanks]

If is a posi don't forget to put in the additive in with the oil.
Check with your Chevy dealer to buy it

[ko-lek-tor]

Thanks for the info. Is there a particular grade of rear end fluid I should buy? Mnay thanks Nigel

http://www.camaros.org/forum/index.php?topic=11234.msg87941#msg87941

[z28z11]

As long as the oil meets the intended spec, just choose your favorite brand. The pics below are out of the '69 Chassis Service manual.

I've historically used Valvoline, and GM posi additive, which I think is still available from your local GM dealer (although the formulation has changed from what it was many years ago). At some point in time, GM changed from whale oil to either a petroleum blend, or semi-synthetic likely by now. Either way, pos clutch plate life (and performance) depends on the correct additive and amount.

Regards,
Steve

[rszmjt]

Personally I like to use Lucas 85W/140 conventional gear lube plus GM 992694 posi additive. I like the extra shock loading the 85/140 offers, I don’t like synthetic gear lube in the diff as it always seems to sweat from everywhere, pinion seal, axle seals, and the tubes. Have you mechanic use a good quality gasket and use a thin film of silicone both sides of gasket to avoid any leaks or sweating.
I’m thinking it takes about 3.5 liters/quarts of oil plus a additive.

[crossboss]

As mentioned, use good 'ol dinosaur gear oil. No synthetic. Oil weight 75-120 is a good choice. IF it has a posi, use a friction modifier for the clutches. BTW, the best friction modifier is made/sold by Ford….really!

[Nigel]

Thanks to everyone who has responded to my post, I really appreciate the response.

When we jacked the car up to turn the wheels we found the original GM sticker on the rear end advising to add posi traction lubricant. Brilliant. We turned the wheels anyway and yes, it is a posi traction rear end.

Thanks for all the info on rear diff fluid and additive, I now have some on order, arriving shortly.

Best regards, Nigel

[Fred Mertz]

If you have upgraded to a Tru-Trac geared posi. A std.80w90 is all you need.  I'm using Valvoline's 75w90 with no issues.