1994 Ford Thunderbird low oil

Engine Mechanical problem
1994 Ford Thunderbird Two Wheel Drive Automatic 120000 miles

I have in dash oil gage it norm. When I am driveing but go's to 0 when I am at a stop light I replace the sender unit an still doing the same I hook up a mechanical gage an I am 50 when cold an get 20 when driving when hot an when I stop at a light get 0 I was going to try puting a oil on it it started low oil press. About 3 week now I run good an don't have no engine sounds thank you