Lately I noticed my lil sohc-tec motor has been dragging and my gas mileage is going down. I changed the air filter, used injector cleaner, and changed the oil. My little 1.6 is still lagging. I took a look at the plugs and noticed that they had some oil on them. - Where did this oil come from? - Is it causing my problem? - What do I need to change besides the spark plugs?