I did compression tests (both wet and dry tests) and found that the cylinder where the spark plug has oil on the thread has very good compression, pretty much the same as the others. One thing that I don't understand, though, is that one of the dry cylinders (the one where plug has no oil on the thread) has 30% less compression than the rest for the dry compression test. The wet compression pressures are pretty much the same for all cylinders. This means that the piston rings aren't the root cause of my original problem. So, it's boiled down to which cylinder has a bad valve seal. The one with oil on the plug's thread or the one with 30% less compression. Man, I'm confused.