If it is on the underside of the oil fill cap, then it is just condensation and nothing to worry about. Pull out the dip stick and see if that looks wrong. If there is "milk" on the dipstick then you have a problem, that meens your head gasket is blown. REPLACE NOW!!! (have you lost power?) also check in the coolant tank. if there is water in your oil, then there is oil in your water/coolant. So the coolant tank will look milky. That is bad. Like I said though, if it is just "peanut butter" under the cap and nowhere else, then it is alllllllll good.