The autoignition temperature for engine oil is between about 300° C and 350° C, so by the time it becomes an issue in your engine, you've probably got bigger problems!
It's much lower than that - Flash Point is circa 220-230ºC, but as the water content of the coolant doesn't turn to steam due to being pressurised, and your fan kicks in around 85-90ºC, you don't really need to worry about it.
