What would cause a 94 thunderbird 3.8L to lose oil pressure at idle after the car has warmed up the car has 158000 miles and its leaking oil rapidly oil sending unit failure?

There are a few things to consider here.

First off, all cars, all makes and all models "Lose" oil pressure at idle when they warm up due to the fact that the oil becomes "Thinner" I Have a 88 thunderbird 2.3L in my mustang and cold at idle it runs about 50-60 PSI of pressure. After it reaches normal operating temp it runs about 15-20PSI at idle. The Biggest thing with oil gauges in newer vehicles is that they are what u call dead zone gauges. They read in certain areas of ur gauge based in the voltage delivered to it by the sending unit through a type of internal electrical gauge which could also be the trouble. Car manufcterers started to use these style gauges b/c customers did not like tha fact that the gauge moved all the time, (the way is was supposed to) but that's another topic...Anyway. My engine showed low pressure for a while b/c a bad internal gauge...i installed a new sending unit and a new instrument gauge cluster i had lying around but it didnt look like it fixed the issue. In order to see if i really had low pressure i went to my local Auto Parts store and bought a Manual (analog) pressure guage, i got a Sunpro brand but many other compnaies make such gauges...in the end i just by passed the electrical oil gauge with the manual gauge and low and behold i had beautiful pressure...it appeared that i didnt because of a bad internal gauge...Long story short, if it only shows bad pressure warm at idle chances are that it is normal or u may have some kind of bad electrical guage if it always seems to have bad pressure....the cheapest way to check and try to fix this problem is to find and remove the electrical oil sending unit in ur car and by pass it with a manual gauge and just go old school and see if ur trouble is from a bad electrical component. Sometimes, in my experience at the least, going with the simple and old school like a manula gauge is the best and easiest way to find or fix a problem with a car.

i also have a 94 Ford Thunderbird and i had that problem with the oil pressure come to find out my oil pressure gauge in the car was wrong i had to run a new oil pressure gauge and it runs just fine now