From 'Roadrunner Engineering' >
http://www.roadrunnerengineering.com/TechTips.html
"As many flathead owners have observed, the idling oil pressure for a warm flathead is often 10 psi or less. While 10 psi is sufficient at idle (it should rise approximately 10 psi per 1000 rpm) it is not very comforting to those used to the higher numbers from modern engines."
"Melling Engine Parts manufactures a high volume oil pump for the flathead designated the M-15 which delivers 25% greater volume capacity than their model M-19 which is the stock equivalent. Peak oil pressure is set by the oil pressure bypass valve (either in the pump or in the engine), however the higher capacity will produce significantly higher oil pressure at low engine speeds. To use the high capacity oil pump in 1948 and earlier engines, a truck oil pickup tube assembly, P/N 8RT-6615 is required. The 1949 and up engines can use their stock oil pickup assemblies. Some bending of the pick up tube may be required to assure adequate clearance (about 1/2in) between the oil pickup screen and the oil pan. The oil pressure relief valve in the center valley of the older engines is usually left in place even though the new pumps have a built in relief valve."