Ok, when my truck, '99 1500 4x4, is cold the oil pressure at idle and cruising speed (70 MPH)is in the normal ranges. Lately though when the engine reaches it's normal operating temperature the oil pressure at idle is right below 40 and at 70 MPH is just barely above 40. Anyone know what's going on. I change the oil regularly and don't notice the oil to be any more dirty or burnt than when the truck was new (now has 57K miles). Why would this only happen when the engine reaches operating temperature. Thanks for the help.