Tim did not mention another "benefit" of the air breather slots, which is that if you are parked on a hill or an angle, and the cowl vent drain hose drips onto the cover, you can have water in the engine! Sometimes I have seen these hoses broken off so that water will get inside even if the truck is level.
Your enigne does have to breathe. That is why I have converted to pcv on some of my trucks, as an alternative to the vented cap. The vented cap has been known to rust out, and I have seen some that were like a pepper shaker spewing rust from the mesh into the rockers. You may find a better quality one with stainless mesh. Logic would have one think they would be made. Wartime (WW2) trucks had a miniature oil bath breather for the rocker cover on the splash 235's equiped with pcv.
