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From Another C3 Forum:

My '78 is occasionally making a loud popping sound and it sounds like it's coming from the rear end. I have heard members say it needs something called a "friction modifier" added to the lube. My question is "what is a friction modifier and where can I get it"?


Positraction rear ends require a friction modifier additive added to their 85W90 GL-5 gear lube. It prevents the clutch plates from chattering and it's available from many E-Bay vendors:


C3 rear ends require two bottles of the additive. Friction modifier additives eventually lose their ability to prevent clutch plate chattering so when changing the rear end lube also add two bottles of the friction modifier additive. I put the additive in first and then I put the gear oil in until it reaches the bottom of the fill hole.
 
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