Defective valve or seals guidelines, oil sneaks from above through the valve offspring and falls inside the compression chambers. Fill everything with carbonilla and admission collecting stain inside. But I don't blow for the vent !! With which you go crazy until you give the failure.
Anyway, these seals are changed when you are cooked to avoid this problem, just as the cylinder head is planned, it is checked under if there are cracks, the slack of the valves of the valves inside the guides is checked ... in the end, rectifying works when you have to lift a cylinder head, the same as if you disassembled pistons, it is normal to put a game of segments (piston rings) connecting rod and bench, which does not represent a very large expense but can cause you a very fat problem.
All the best.