94 3.8 gas in oil

hello guys. my mom has a 94 mustang with the 3.8 liter. recently her MPG has been more then cut in half. shes getting HORRIBLE mileage compared to before. this has been going on for about a month now. i went to change her oil last night and about 4 gallons of fluid came rushing out of the crankcase, it was a watery type substance and reaked of gasoline.
needless to say, her gas is getting into the crankcase and mixing with the oil.. my question is, for the people with all the mustang experience, what are some things that could cause this?

There are a couple of things that could cause this. First, the head gasket could be blown allowing the gas mixture to be forced into the engine oil. Secondly, and this is common to 94, 3.8L six cylinders, the fuel usage and drain down into the crankcase has been known to be the result of a faulty computer that is telling the injector(s) to stay open. This in turn is flooding the cylinder with gas and gas which is pooled into the cylinder is draining down into the oil.