It's hot here in Florida, yesterday 95F (feels like 102F).
On my 1967 Volvo 122S B18 Eng., last oil&filter change used 15W/40 oil

Yesterday, driving along at 60-65mph; air-cond. on
RPM; 2500
Eng.coolant temp. 180-185f
Oil pres. 28 - 30psi
Oil Temp. 210f
But, when coming to a stop at a stop light, oil pressure drops to 0/zero, oil pressure light comes on, unless I raise idle RPM or turn off the A/C.
Is this normal ?
Is my oil pump going bad ?
Would a straight weight heavier oil such a a 50w help ?
Any advise would be appreciated. Thanks

Most likely cause is one of the oil pump O-rings has partially blown out. Keep the revs up to atleast 10psi and fix it very soon. Left alone, the eventual outcome will be a spun big end bearing.

I use a regular O-ring behind the square section O-ring to reduce or prevent this problem.

After connecting a mechanical oil pressure gauge still same readings, barely 30psi on start up with a cold engine. Will now look for a shop that can possibly lower the front cross member 4-5inches so that the oil pan can be removed and oil pump and seals can be replaced without pulling the engine.

I've done that with car ramps, multiple jacks, etc, and wish I would have just yanked it out the top the first time. You are going to want to look at the big ends too see how much damage has happened also.

I agree.

It's a bit more work to get the engine out, but once you have it on an engine stand you will appreciate the extra effort by the ease by which you can work on the engine.

Laying on your back under the car and having oil dripping on you is your choice.
--
Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com

I agree.

It's a bit more work to get the engine out, but once you have it on an engine stand you will appreciate the extra effort by the ease by which you can work on the engine.

Laying on your back under the car and having oil dripping on you is your choice.
--
Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com

Since most wear occurs during start-up a heavier oil will cause other issues.

Your great looking car deserves the options suggested below.

No, that isn't normal.

15W/40 should be fine, just make sure you're getting enough ZDDP to protect the cam and lifters.

There are numerous reasons for low to no oil pressure.

It could be as simple as bad oil pump seals as I won't use ANY of the aftermarket seals, only Volvo original oil pump seals.

Or it could be the oil pump, or badly worn bearings.
--
Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com

Thanks Eric, Engine was freshened up about 45,000miles / 24 years ago with honing of cylinders, new rings, bearings, valve job with hardened seats. Recent compression check showed 160-165psi across all cylinders. So I assume that I need a new oil pump and seals, or maybe for an upgrade to a B20.

Maybe before going deeper check the accuracy of your sender and gauge at low psi.

In the next days, I will connect a mechanical oil pressure gauge and advise the outcome.

Use a good 20/50 like most B18/20 owners. Yes something with high zinc helps. Dino oil loses its ability to maintain viscosity at high temperatures surprisingly quickly. By about halfway to the 3K oil change it has started to drop noticeably. Makes it hard for it to cope with your current Florida temps.