2001 Chevy 3500 gas 8.1 dualy lifter knocks - oil not staying in engine lifters are knocking.

2001 Chevy 3500 gas 8.1 dual excab; lifters knock after changing the oil pump, this didn't fix the lifter knock. Lifters are knocking at cold start then clear up after 1 to 3 minutes. Oil pressure goes from 20 to 60. 20 at idle and 45, or 50, while driving. All after a 2000 mile trip. Need info on associated sensors, recalls or bulletins. Trying to find sensors for cam, knock or crank testing. Need to know if anyone else has heard of this problem. Answered 6-17-11;
. But need additional info 6-18-11. Need info on the oil check valve that holds the oil in the engine? Any known bad after market parts? Any known oil check valve part number?. Is the oil check valve in the oil pump or an additional part? Thanks.