i have found that a PCV valve that is not right can cause to much water to be in the crankcase. so change it once in while. all the acids in your oil are water based. and for every gal of gas you burn almost a gal of water is made.
Back when I was in school, my instructor told us that the PCV valve was pretty simple. More or less a check ball system to prevent fluid oil from getting through but still allow vapors to pass. When they stop working it's usually because gummy oil residue has stuck the check ball. I was told that inspecting them was a simple matter of taking them off and shaking them. If the ball seemed to move freely back and forth without any sign of sticking, they were OK. All we ever had to do to fix them was to clean them in parts solvent.