Why is there gas in the oil of my 2000 dodge neon?

I noticed a sputter in the car last week then my check engine light came on, So I went ahead and changed my plugs and wires. The light stayed on and when i checked the oil its thinner than it should be and smells like gas, my first thought was that this was because of the bad plugs and wires(the sputtering stopped once they where changed) but a guy who works at the part store told me that bad plugs wan't cause gas to get into your oil, so what could it be