I have a 2003 Volvo 770 with Cummins ISX 450HP engine 860K miles. A few months ago I nottice that my coolant exapansion tank is turning a little black inside and I though it's just gunk from the engine running hot during summer with heavy load in California.
A few days ago I pulled the coolant filter out and surprize, a few drops of oil inside. In the past few moths I did 2-3 oil analisys and everything was great, no coolant in oil. Oil pressure it's ok, water temp ok, coolant level with engine cold about 1/2 of tank with engine hot full tank and some coolant in the left side of tank. There are no air bubbles in the expansion tank. No white/blue smoke, nothing besides soma blackness in the coolant tank.
Does anybody know how this small amount of oil got into my coolant???