300E 91 150K Mi light brown oily liquid floating in coolant tank

[Papatatun]

Engine is running perfectly with stable, normal idle.
No emulsion visible on oil dipstick, only engine oil, dark brown.
Transmission dipstick shows very clean light brown oil.
Flushed coolant with water hose open in coolant tank, engine running, letting water out through open radiator valve at bottom: some tan colored sticky liquid came out through the bottom, then stopped. After 15 minutes more flushing, closed the radiator drain valve and used the car again...next day: some of the same light tan colored liquid was collecting on top of the coolant in the coolant tank...
Can this oil come from the transmission radiator built inside the coolant radiator, and if so, how can I check for this?
Or can it still be the head gasket failing, even though the engine runs fine and there is no sign of water on the oil dipstick? If so, is there a test I can run to help narrow it down to this problem?
THKS.