If the miles are up... try switching to a 20w50 or 15w50 synthetic oil.

It is not clear whether you meant the 20 psi was at idle or normal running.
20 psi at idle is fine. As someone said, most warning sensors are set to cut in at 5 psi.
But if it never goes above 20psi when running then that is very low and you should investigate it and get it fixed.
Check firstly the sender, the wiring and the oil filter, and make sure the gauge you are using is accurate. But if it still never gets above 20 then it will be a worn oil pump, or very likely the rubber seals between the pump and the engine block. These harden with age, and get partly pushed out of place by the oil pressure. (Take measurements with the oil hot.)
There is an excellent picture somewhere on this site in the FAQ. I had your symptoms, slowly getting worse, so I had a look. Sure enough, the seals were going. After replacing them and the pump the pressure is now 30 psi at idling and 50 psi at 2000 rpm. Previously it was virtually nil at idling and struggled to reach 20 at any speed.
If you do decide you need to investigate, I have a trick for getting the sump pan off. Let us know how it goes.

A good rule of thumb is 10 psi per 1000 rpm.

Not sure about the Volvos, but most oil pressure lights are set to trigger at 4-5 lbs of pressure.

What weight oil are you running? When was the last time the filter was changed?

My cars generally run about 25-30lbs at idle when warmed up.





--
don hodgdon '89 744ti, '81 242t, '71 D-35

Check to make sure that the wire that goes to the oil pressure switch (I believe it is on the passengers side of the engine behind the alternator) is not shorting to ground / grounding out. When the wire gets exposed to the heat from the engine and all the grease and gunk from all those years of use the wire insulation crumbles a part and the exposed wire touches ground somewhere thus the light comes on. Good Luck!

Sincerely,
--
Julio Meza

1979 264GL B27F 124K --> BW55->M46, CLS, IPD Sway, 81+ Dash, Gauges, Next: FLATHOOD

1979 262C B27F 159K --> Restore Project w/Possible B21FT

1984 245DL 212K -->Wifemobile