oil leaking between transmission and motor

Hello, This is most likely the result of a failed rear main seal for the engine crankshaft, or, if the fluid is actually transmission fluid, if could be the transmission front seal. Sometimes a valve cover leak can look like a rear main seal, and an oil pan gasket can do the same. Inspection is needed to identify the cause. Also, consider when it is leaking. If it only leaks while it is running and just after, it is not likely the rear main seal. If it leaks all the time, it is not likely the oil pan.