Oh crap. More bad news today. It really seems to be an internal problem. I think I need to visit a repair shop.
I cleaned the oil in the plug holes, the car was fine for a while, but then, after I stopped it and restarted the engine, a spark plug was not working and lots of smoke was coming out of the exhaust pipe. I stopped the car shortly afterwards and all spark plugs were
full of oil (but none was dead). Eitherway I changed them all, but the problem was not immediately solved. I let the engine off for 20 minutes and when I restarted it all plugs were working. After some running the smoke stopped coming out of the exhaust pipe as well.
And this is something I've noticed before but never paid attention to it: I never had any problem to occur while the car was running. All problems appeared right after I stop the engine and restart it. And in fact, the engine does take 4-5 times of trying in order to start.