here is a trick to get oil out of your radiator once you find the problem. Put clear water in the radiator once you get the water warm by running the engine add auto matic dish washer detergent ( gel works best) . Run it for a lon g time then drain and add anti freeze. The auto dish detergent is castic soda based so it doesn't foam up. It will get rid of any
oil in the water.
This works real good in diesel engines with leaky oil coolers. just a tip.