My son has a '97 Jetta. There's milky stuff on the oil fill cap and also a tad on the end of the dipstick. Water, I assume. We've done a number of checks. The car doesn't get hot. (Thermostat is missing or stuck open, so never really gets even to temp, but that's another issue). No white smoke. Compression is good. Really no indication at all that there is a head gasket problem. Where else could this be coming from? I seem to recall that the heat exchanger for the oil (where the oil filter is located) could cause this. How does one test that?