Yesterday while driving home I noticed that when I am just idling at a light or push in the clutch my pil pressure gauge drops to zero. I stays between 20-40 while driving but once it warmed up it drops to zero at an idle. It seemed to stay at about 20 when idling cold though. It is an '89 with 201,000 miles. Do to leaks I have replaced the rear main, oil pan gasket, valve cover gasket, and the oil filter o-rings. The slowed down considerably but still leaks a little and now I am having this pressure issue. Any ideas what might be going on and advice as to what weight oil I ought to be using as well. I live in Denver if that makes any difference.