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      10-09-2012, 08:31 AM   #37
davesaddiction
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Drives: '08 E90 M3 6MT SSII BPM Stg II
Join Date: May 2012
Location: Tulsa

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Quote:
Originally Posted by SenorFunkyPants View Post
Fair enough, its your car and your choice....I choose not to have the extra slip oil.
Just so everyone is clear as to what is happening:

"The Variable M Differential Lock recognizes the differential speed building between the driven wheels and generates pressure in an integrated shear pump. This pressure activates a multiple plate clutch via a piston, and conveys drive forces to the wheel with better grip, according to the difference in wheel rotation speed. In extreme cases, the entire drive forces may be transmitted to the wheel with a better frictional coefficient."

The rubbing groaning noise you hear usually with cold diff oil and in tight turns is the noise made by the steel clutch plates held together in a pack slipping against each other. These high friction clutch plates have to be able to transmit some 350bhp without slipping to one wheel at one extreme and yet slip relatively freely when unloaded to allow the car to make a turn.
The question is, does adding an extra slip agent to the oil to enable the clutch plates to slip easier and thus quieter in a unloaded turn effect its function at the other extreme.
All I know is my car makes a little noise in tight turns when cold but locks fully when needed - I'm not going to risk that by adding a non OEM oil.
Thanks again for the in-depth explanation. Makes me feel a lot better to know that if I don't get the fluid changed, I'm not doing any damage to the system. Sounds like it's a fix for "customer satisfaction", and nothing more.
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