Come spring when my M3 is done with winter storage, it'll be due for a rear differential fluid change. The rear diff comes with a GKN Visco Lok, which is a speed-sensitive LSD. Factory fill is Castrol Syntrax Limited Slip 75w140 with additional friction modifier -- more on that below.
When the E92 M3 was first released in 2008, people complained about a noisy rear differential. Specifically, there was some chattering at low speed when the steering wheel was at full-lock (think parking lot maneuvers). Obviously the fluid had friction modifier already, but BMW decided to up the FM content to address customer complaints. If I recall correctly, the service bulletin had a recommended amount of FM to add, but also mentioned it should be done "to customer preference". If you go buy a bottle of it at the dealer, it's actually private-labeled for BMW and has the additional FM in it.
My question is this: doesn't additional friction modifier actually reduce LSD locking? If that's true, isn't the best performance gleamed by NOT using it? Are there any other downsides to less FM other than the noise? I understand BMW has to balance performance and driveability to keep owners happy, but I do track the car so performance is important.
There are a few fluids that do not contain any FM whatsoever, and of course the vast majority of differential fluids contain the "normal" amount of FM. Which avenue should I take?
(FWIW, the fluids I'm considering are Renewable Lube, Gulf Competition LS, Torco SGO, and the non-BMW Syntrax LS)
When the E92 M3 was first released in 2008, people complained about a noisy rear differential. Specifically, there was some chattering at low speed when the steering wheel was at full-lock (think parking lot maneuvers). Obviously the fluid had friction modifier already, but BMW decided to up the FM content to address customer complaints. If I recall correctly, the service bulletin had a recommended amount of FM to add, but also mentioned it should be done "to customer preference". If you go buy a bottle of it at the dealer, it's actually private-labeled for BMW and has the additional FM in it.
My question is this: doesn't additional friction modifier actually reduce LSD locking? If that's true, isn't the best performance gleamed by NOT using it? Are there any other downsides to less FM other than the noise? I understand BMW has to balance performance and driveability to keep owners happy, but I do track the car so performance is important.
There are a few fluids that do not contain any FM whatsoever, and of course the vast majority of differential fluids contain the "normal" amount of FM. Which avenue should I take?
(FWIW, the fluids I'm considering are Renewable Lube, Gulf Competition LS, Torco SGO, and the non-BMW Syntrax LS)