1994 Nissan Sentra Car sputtering and smoking

I recently changed my spark plugs and valve cover gasket, on my 1.6 motor. Every since then at low rpms the car wants to sputter or have no power like it's not getting gas, then it's folllowed by what I think is white smoke. But by reading ur site white smoke is coolant but this definatley smells like oil coming outta the exhaust. But holding hand over tailpipe leaves no oil residue. The plugs I bought and metal rings u screw onto the plug before putting in motor and we either lost one or one didn't come with the ring so we took one off the old plugs, only problem being they didn't seem to be made to come off. So we used pliars and forced the ring off and then onto the new plug. The ring was completly intact but was bent some while trying to get off the old one. Was wondering if the plug or maybe we didn't seal the gasket well enough could be causing this problem?