I think it is engine oil since it is brown, and not transmission fluid. Very minor leak right at the circular ring type area where the engine meets the transmission. I am assuming that this is not really a "wet" interface. So is the leak from the "rear engine seal" or the "crankshaft seal"? Do those seals mean one and the same thing? Or are there two different seals in that area?
Extremely minor oil leak near engine and transmission interface? on 2004 BMW X3