put ya fingers in the bore is it scored at all?

pull it over make the piston come back even with the block and wiggle it see if you get much play?

pull it over and see if the valves are sticking?

just scrap of gently the sticky gasket and get a new one. about £8-£12 from that then it could be a factor yes. i never used to get many gaskets go tbh. and ive serviced thousand of them fuckers when was it serviced last i used to get customers that self serviced them and the head bolts werent dont up tight and that gives the problem too.

oil on 3 of them bolts and in your exhaust could be due to the ride-on hase been tipped slightly on its side cleaning underneath or something or over filled but more the 1st bit. dont worry will just burn off (you will get smoke just let it burn and make sure level is ok)

is the filter oily? if yes wash out with fairly liquid or clean with petrol and blow with air line.

and no dont worry about skimming