Hi Guys.

On my previous post, whining in the rear end of car, I just found out that the previous owner put synthetic oil in the differential. Do you think that could cause the noise in the rear end of the car. Do you think I should go back to the original oil.

Advice would be appreciated.
Thank You,
John.

I don't know about the other members,but I'm not real sure about synthetics,I'm from the old school ,I use the real stuff in my 78,plus 2 small bottles of additive in the rear end. thats my choice,you'll probably get all kinds of opinions on this but thats the way i'd go

Personnaly, I only use synthetic engine oil. In the rear diff tho, I'm not sure if the GM additive is compatible w/synthetic, it's probaly NOT. The p/o probably didn't put any additive in. I'd go back to regular GL-5 (80w90) with the additive.

Synth gear oil can be a good thing. The problem is with the friction quality. Due to the positraction, the friction/lube quality is very important. With out the proper additive, the posi will grab and jerk, and make a noise on corners. With too much slip, the posi won't grip and function, allowing one wheel to spin.
In a straight line, it won't make any difference.
I don't know of any specs comparing newer synth oil to the regular stuff with the posi additive.
If you don't have a positraction, the synth in the correct weight should be okay.
Straight line noise in the rear end is more likely the ring and pinion gears, or bearings. This could lead to a big problem no matter what gear lube is used. Just be sure it actually has gear lube, and not something else.

Ken Styer

|UPDATED|2/1/2004 5:51:01 PM (AZT)|/UPDATED|

Can anyone describe the symptoms of not having the correct additive in more detail? I suspect that I may have that problem.

Thanks!

When driving in a stright line there will be no symptoms. But when turning, the clutchs in a posi need to unlock and slip. Without the additive, they don't slip properly and grab, then release.
The symptoms will be felt by a jerking feeling and a popping noise on turns. This can be very minor or get to be so severe that the car can actually bounce to the point of feeling like someone hit you in the rear. The correct fluid and additive will make the problem go away.

Ken Styer