An oil catch can goes in the PCV hose, and catches the oil mist, preventing it from going into the intake manifold. The oil gets burned away, so it's not like it sits there, but it can carbon up the valves. I put one on my truck at about 1,000 miles, because that's when I learned about them. At just over 8,000 miles, it has caught about 5 ounces of oil, that would have gone into the intake, to get burned. I imagine that will slow, when the engine gets more broken in, but I like the fact that no oil is gumming up the engine.