Skoda octavia mk1 thin film of oil

I have a 2004 skoda octavia mk1 l.6 and I had oil mixing with coolant problem it was the transmission oil cooler so I changed it and the engine oil cooler as well. By the way the engine does not over heat and there is no signs of bad head gaskit. Now when the car is cold in the morning I see a thin film of oil in the coolant resevoir. I flushed without adding any flushing fluid, only keeping the engine running, open the bottom drain plug and keeping the hot water drain . I did it  multiple times but this film keeps coming back. Could the coolant system still be  contaminated with transmission oil. Should I flush it more with a flushing fluid . Also does the transmission and oil coolers fail immediately or leak at a slow rate.