1991 Chevy Cavalier Low oil pressure when warmed up at idle

Engine Performance problem
1991 Chevy Cavalier 4 cyl Automatic 123000 miles

hi there

wondering if anyone may know here what may be the problem with my '91 chev cav.

10,000 miles ago "check gauge" light comes on at idle. Then the battery light started coming on also a week later only at idle. I got everything checked by mechanics and resulted to a faulty gauge. I also had a mechanic install a rebuilt alt just to be safe and he changed it 2-3 times because the light still came on. After a couple months of driving the battery light doesn't come on anymore but gauge light still does at idle and letting off accelerator.

the 6 years i've owned car the oil pressure gauge on dashboard stays to max regardless if engine is warmed up or cold and at idle or speeding. Mechanics said nothing is wrong. 1,000 miles ago now the oil pressure gauge goes to the lowest after engine is warmed up at idle and on speed only goes up only 1/8 way to 1/4 when car is warmed up.
it has ran perfectly fine, so mechanic said nothing is wrong then and may be a faulty oil pressure gauge since it's old, but no need to replace since engine runs fine or I can just spend $60 to see how the oil pressure gauge does then.
i did however have him change the oil and install a brand new battery that was now needed.

now today. Car died on highway while driving 55mph. It wouldn't start when I pulled over. Then I immediately tried again and white smoke came out of engine area and the engine wouldn't stay on. 30 second later I tried to start it again and it started this time and ran normal. No smoke, no stalling, no noise. The same oil pressure being low though. I got off highway and drove back streets for 10 miles to get home.

i checked under the hood when I luckily got home w/ car and everything looks fine by apperance but oil was low by 1/2 quart. I didn't make sure he put enough oil in after the change last week. The car has never leaked oil before, but I haven't paid attention if it has lately. I assume it hasn't.
air filter is a little dirty but not terriable.

it's the original engine. 123,000 miles. And i've kept the car in great working order. Runs new basicaly.

thank you for your time