Q: My car is leaking oil after an oil change.

I have had the car for less than one year purchased with 128k miles. I have had the oil changed twice. The engine was dirty and some visible oil near fill cap but brushed off as spills. Although after this second oil change I have noticed a distinct burning oil smell and now have put another 1300 miles since the change. I have kept checking the oil level and it is still full but I am sure the oil spill should have burned off by this time. I was going to change spark plugs today and when I removed one plug I seen the have a considerable amount of dark colored oil on it even on the threads and it was kind of slimy (stretchy like saliva). I went ahead and put the new plug in and cannot get the rubber seal (assuming something like a valve seal that can slide up and down the wire itself) to fit down inside the hole anymore. Not sure if it is actually supposed to fit inside or around the outside. Any knowledge or advice is greatly appreciated.

My car has 137000 miles.
My car has an automatic transmission.