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I don't go by any set number, I've found each differential's limited slip clutch pack is different. On the first lube change, I like to add a single bottle and drive it a few days. If the diff needs more fluid, I'll hear the clutches clucking when I back the Vette up and crank the steerng wheel hard.
My Impala needs two bottles, my Vette needs only one. I want to add just enough friction modifier to prevent the annoying clunking but have the limited slip be tight.
Heard this about the RelLine brand before,... not sure about the RPurple. Either way,.. after a recent talk w/ diff people @ Reider Racing,... they said to Not use synth fluid.
The additive is avail under the CRC brand name,... any auto parts store,... less $. Don't have the P# w/ me. Large plastic toothpaste tube,... say's the Fud & GM P# right on it. Ask the counter guy.