coolant in the oil, or oil in the coolant.

do a compression test as well. typically, if a head gasket is blown between adjacent cylinders, the compression will be the same for those two (same, as in, low results, but equal to each other).

example: if you get the following results.. 180, 160, 160, 180.. the middle two cylinders possibly have the head gasket damaged between them. this is also assuming that you don't have worse problems, like worn rings.