Like I said, I hadn't really thought about the "why's" when my oil pressure improved with the crankcase vent but Jerry's post made me realize that all I was seeing was a change in the indication, not the actual pressure. At idle I was pulling a vacuum on the crankcase, and my gauge was indicating about fifteen pounds. With the breather the indicated pressure rose to between twenty and twenty five which makes perfect sense. The pump was delivering at the same pressure, but the indication was looking at the overall pressure across the pump and without the breather my suction head on the pump was right at negative eight pounds, 16 inches of mercury. Pull your oil filler plug and see if you idle oil pressure jumps up, then decide if you want a breather or not.