I have Chev Cruze 2010. There was a heatup and I noticed thick creamy sludge overflowing from the coolant reservoir lid. Mechanic diagnosed issue with engine oil coolant and repaired it. Later he also changed the transmission oil coolant. After the repair, the car is not heating up anymore; however, I still get the thick oily sludge in the reservoir (please see picture). Mechanic says it will take some time before all the oil in the radiator is flushed out and that I will have to get the coolant reservoir cleaned after every week or so until all the oil inside is cleaned up. I am not sure and am doubtful whether the issue still exists? There is no coolant leakage into the engine oil as engine oil seems just normal. What should I do?
The tick creamy sludge is caused by oil and coolant mixing - so you need to know the original cause of that mixing - i.e. how is the oil getting into the coolant in the first place. The most common causes of this are failures of headgaskets or oil coolers.
The suggestion that it will take some time for all the oil to be flushed out is rubbish, as any competent mechanic will have thoroughly flushed the system through before refilling it.
I'd suggest finding a new mechanic...