Oil Pump or Rear Main Seal Leak

If any fluid leaks between the engine and transmission, check whether it's red or brown. If the fluid is red, it is automatic transmission fluid, and the transmission's oil pump seal is most likely at fault. Always check the transmission fluid's color because ATF will turn brown from heat and oxidation.

Rear main seal leaks are common; if the color of the fluid is brown, it's most likely engine oil leaking past the rear main seal. The rear main seal prevents oil from leaking past the spinning crankshaft flange. This leak is a common problem as an engine wears. If either seal leaks, the engine and the transmission must be separated, and a new seal must be installed to repair this leak.