It's mayonaise! That's what we used to call it when oil and water mix in the presence of an emulsifier. Any acid would do for the emulsifier, as well as some of the additives in the motor oil. If you make trips shorter than 20 minutes in length, have a bad thermostat that isn't allowing the engine to warm up fully, or have a head gasket leaking, you may have this problem. I always heard that early versions of M1 were not too good at keeping water in suspension, therefore allowing it to collect on inside engine surfaces, but I thought that problem was solved by the SS. I hope your problem is a cheap one to fix, the end result will be sludge in your engine if it continues.