I was thinking that but what about the sluddgy mayo when your head gasket is gone?
Oil and water will mix to make an emulsion (like mayo), but it requires an emulsifier. I did a job once treating the run off from the ford engine plant in Bridgend, all the run off from a few square miles is captured and treated before being let into the wild. It makes a pretty gruesome emmulsion. It'll seperate again if stood long enough.
If you see someone cooking and adding a bit of flour to a sauce it can act as an emulsifier and bring two immiscable liquids together, and bring a seperated sauce back from the dead. I'm assuming with head gasket mayo some other deposit in there creates the emulsion.
en.m.wikipedia.org/wiki/Emulsion
Only thing I know that can mix oil and water into custard is a rover K series engine id like to see someone try it with a bucket and a stick.