Are you using the oil filled gear diff mate? I tested it last night in the m06 with kit 2000wt oil and it was way too heavy so will need to be 750 or 500 max I would say. Over on the tamiya mini cooper thread there is a few posts about it at the moment if you go back a page or 2. I am going to try again with thinner oil though. As a rule though I would have thought it would need to be way thinner than what you would use up front. Hope this helps.