In terms of oil in the cooling system, my experience suggests that the combination of oil and coolant produces a thick sticky sludge, which may be inclined to coat surfaces, and perhaps clog things up. About 3 flushes with just plain water seems to get the worst of it. The oily sludge tends to float on the water, so it will show at the top of the radiator, and settle into the expansion tank. The tank needs to be cleaned thoroughly as well.
Perhaps accelerate the next coolant change. After that, the residual oil should be nearly gone.
1986 300E 5-Speed 240k mi.