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      08-21-2012, 03:47 PM   #17
Z K
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Drives: E90 M3, MK6 GLI
Join Date: Jul 2011
Location: San Francisco

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You only need the FM (aka Friction Modifier) booster if your diff is making grinding noises. All it does is make the noises go away. Otherwise you can use the regular LSD diff fluid which is a lot less expensive.