Engine misfire help?

I have a 1997 Cadillac Seville STS with 128,000 miles on it I have a constant engine misfire enough to create a small vibration while driving at high speeds and enough to fail smog and make my gas mileage drop down to half of what it should be and at the same time every once in a while the check wngine light comes... show more I have a 1997 Cadillac Seville STS with 128,000 miles on it I have a constant engine misfire enough to create a small vibration while driving at high speeds and enough to fail smog and make my gas mileage drop down to half of what it should be and at the same time every once in a while the check wngine light comes on for a faulty o2 sensor but it doesnt come on for anything else I replace the fuel pump fuel filter all the spark plugs and wires and it seems that I have a small amount of oil in the cylinders that I found out when checking the spark plugs the spark plugs had oil on them I believe that to be causing the misfire but I can't find a oil leak and I can't figure out how it's getting into the engine cylinders
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