oil pressure problems on a '40 series 40



I have a '40 series 40 with about 64,000 miles on the original engine. I recently got the car running and did not do any internal engine work to it. When I hit the road and get it warmed up good, the oil pressure drops to about 4psi at idle and that sounds real low to me. My first step was to verify that the gauge on the car was correct, so I installed an aftermarket gauge and got the same reading. I droped the pan, cleaned out all the gunk and cleaned the oil pump out. I checked the gears on the pump and had .008 to .009" clearance between the gears. I do not know what that dim should be. Put it all back together and filled it up with 15-40 oil, and it still drops to about 4psi. The car runs a good 40 to 50psi when you first crank it up and about 25 to 30psi at 55mph warm. Is it normal for these old engines to drop that low at an idle? Should I even try a new oil pump or go ahead and overhaul it. The engine is not knocking, runs smooth and is not burning oil, so I hate to tear into it unless I have to. Thanks for any help that you can provide.