I have a 1962 Atlas. The engine has been re built, crank re grind/new shells, re bore with new pistons and rings, new valve seats and guides etc, also new 6 start oil pump from Andover Norton, it has the original low pressure feed to the head. I have also fitted a Holland Norton works anti sump valve and oil pressure gauge. I don't want this to be about the rights and wrongs of anti sump valves, I accept and understand both sides of the argument. However, my concern is this. On starting from cold I get a reading of around 65psi which with 30 of reasonably hard riding minutes drops right back to 10-15psi, even at 3-4000 revs. It has Duckhams 40 mono grade oil in it. Any ideas? Should I even worry? My Commando has had the same treatment and maintains 30-40psi at higher revs when hot.