I have a 90 GT that is for the most part stock except for an o/r xpipe, and mac induction tube. I recently saw my fuel economy drop way off and did a tune up.....oil, plugs, air filter. One of my plugs was a bit fouled and so i checked the new one after 200 miles of driving. the new plug was extremely oil fouled and looked as though it was not even firing. This obviously leads me to believe that there is an oil leak into that cylinder, however, my car is not smoking at all and does not appear to be losing much oil either (it always burned a little). when driving, the car seems to be firing all 8 cylinders and the power seems normal. My MPG went from around 17-19 to 12-14. Any ideas on what the cause could be?