My 5 cyl deasial Engine has oil in my raditor overfow and a small drip of oil comes out now and then. Engine oil looks great very little loss. No overheating or other issues. Had new transmission put in a month and 2000 miles ago no issues. Can it be the radiator or oil cooler?
The oil you see in the reservoir could be transmission fluid. The transmission cooler in the radiator right side tank could be leaking transmission fluid into the cooling system.