Eric; the figures you give are puzzling. If you're getting pressures that are only limited by the bypass valve at high revs, you should be getting more than 10psi at idle, unless you have the idle set way down - 500rpm or less. I don't know about you, but Matilda needs at least 800rpm to idle smoothly, and at that speed, on a
well worn 1725 (e.g. wear steps in the cylinders) I still get 25psi or so with hot 10w40 oil, and bypass pressure at 1500 and up.
I only know of five reasons for l.o.p:
1) Worn mains and/or big ends;
2) Too much wear clearance in the oil pump;
3) Clogged filter, or other blockage
upstream of the gauge pickup (downstream would tend to give higher, not lower, readings);
4) Faulty gauge;
5) Too
low a viscosity oil.
6) Faulty bypass valve;
So:
1) Only you know. And that would reduce pressure all through the rev range;
2) Ditto
3) Absolutely ditto. In fact, this would more likely cause poor pressure at higher revs where the
flow restriction would be more significant than at low;
4) Again, one would expect problems all through the range, and other symptoms like the needle momentarily sticking as it rises or falls in response to a change in revs, or failing to return to zero;
5) Obviously, not in this case;
6) Possible, if the plunger has stuck at the point where just a little of the return port is exposed. At low revs most of the pressure would be lost, but a strong pump output might keep it up at high.
And you were wondering what you were going to do to occupy yourself this weekend