1999 Toyota Corolla Low Oil pressure light turns on

Engine problem
1999 Toyota Corolla Engine Size unknown Wheel Drive Type unknown Automatic 149000 miles

I have a 1999 corolla with 149000 km, which I bought about 2 yrs ago. I did not experience much problem with this car till last week when the check engine light turned on while I was stuck in snow. The light went off on its own after few km. But 4 days later, when I was driving on highway, low oil pressure light started turning on. I pulled off, allowed it to cool, check oil level. Oil level was ok, then when I started driving, the light started coming on after few km again. Went to a mechanic, who replaced the oil pressure senser saying it may be faulty and also said there could be oil pump problem since engine gives different sound. So he put thicker oil (10W30). Everything went fine for short distance, but again when I drove 20-30 km on highway, the low oil pressure light again started coming on. The oil level is still ok, no weird smoke. Another mechanic suggested me to add no smoke stop leak oil additive.
I am very confused on what to do further & worried that engine may stall. Tour valuable advice will be greatly appreciated.