During the winter I changed the oil in my 100K+ engine to 10W40 and, as yet, haven’t changed up to my usual summer 20W50. During my last drive to a local show, the weather was sunny with temperatures in the mid to high 80s. An idle speed of about 1K RPM (getting to a 500RPM idle on my P1BJ8 has always been impossible) produces a 20 lb pressure with 40+ lbs at 2K to 2.5K. Although idle pressure is a slight bit higher when using 20W50 on a hot summer day, running at 2k to 2.5K RPMs does produce a solid 50 lbs.
It sounds to me that you are doing great by getting 20 lbs at a warm 500 RPM idle. You are also doing fine running at 40 lbs.
I have never read any Healey specs that indicate a 60 lb warm road speed pressure and the only reference to hitting 60 lbs is the setting of the oil pressure relief valve. I do hit 60 lbs on a cold autumn morning on initial start up with 20W50 in the crank.
Hope these data points help.
Ray (64BJ8P1)