I have a 2006 impreza 2.5i with 135,000. I did my timing belt & water pump about 4,000 miles ago. Now my car is misfiring on cylinder 3 and has a little bit of white residue on the plug and oil on the threads.. I also have a slow oil leak around cylinder 3. Would this be the headgasket failing?

 

Is there any residue where the head and block meet? Kinda sounds like a piston ring is shot but not real sure