Im doing my buddys 300e right now same situation as you mentioned, i found the oil in the coolant, previous owner said head was cracked but
i found no obvious signs of cracks , while pulling the head all the bolts were too loose, indicating possible prior overheat, measured 2 1/2 thousand warpage, resurfaced head , torqued bolts per manual and all is well, had to change all the rubber hoses as they were expanded from oil contamination , ive drained and refilled about 6 times to get all the milky
residue out , but its finally clean!, at the rear of head an oil gally is close to the coolant passage,so that must have been why oil was mixing with coolant.