Truck is a 2000 Silverado 5.3. My oil pressure while idling is always at 40 or either just one notch below 40. I was driving today and while on the interstate I noticed my oil pressure was low (at 40 while at 2000rpm) when it's normally at like 60 at that rpm. When I would stop and my engine would idle at the normal 500rpm the psi would be at 22-24. This worries me a lot so I drove easy until I got to my destination. When I arrived I checked my oil level, it was perfect. I then cranked my truck up again, oil level was at 40 like normal. When I got back to my truck an hour later to head home I cranked it and it was at 40 as well so perfectly normal. After a few minutes of driving it started going back down again. The lowest it'll be is 22 while idling. Anyone know what's causing this and how I can fix it?