OK here is an update.... I sent this letter along with a bottle of the synthetic oil and a bottle of the additive:
Dear Scott,
I am writing you in response to our telephone conversation yesterday. I brought my car into your shop on the morning of October 12, 2005 to complain of a clunking noise in my rear differential. Upon your mechanics review he noticed that I added aftermarket sway bars to the car. From that he eluded that the sway bars could be a possible problem; upon reviewing another GTO from your lot he decided that was not the case. Then, he questioned the type of oil I use. I explained that I use Torco Gear Oil. To be specific following are the specifications:
GM RECOMMENDED: SAE 75W-140 GL-5 Synthetic Gear Oil
CURRENTLY INSTALLED: Torco 100% Synthetic Gear Oil SAE 75W-140 (Meets or exceeds API-GL-6)
Note: For limited-slip or posi-traction differentials use Torco Limited Slip Friction Modifier Type F (for all applications except GM) or Type G (for all GM applications).
Your response was that if I used the Torco instead of the GM oil it will void my warranty. Not only is that untrue, but it is illegal for a warranty to be voided just because an aftermarket part was added to the vehicle. Under Magnusson-Moss Act a dealer must prove, not just vocalize, that aftermarket equipment caused the need for repairs before they can deny warranty coverage.
As expressed in our phone conversation yesterday, I will work with you to resolve this problem. If you feel that I need to change back to the factory GM Oil to “correct” the problem I will do so, but not at my expense. Either you can change the oil and monitor this at your cost or I will have the last company that changed my oil do this change at their expense, which they have already agreed to do.
Please advise the next course of action.
Regards,
SC
So now they are replacing the oil with GM Synthetic, driving it, and then replacing the synthetic a second time. (I agreed to pay for the lube, but not the labor.) If tht doesn't correct the problem, then they will replace the differential.
So I will know in a few days.
However, the good news is my friend feels bad for me and is allowing me to drive his Porsche Cayenne S until the GTO is repaired.