My name is bruce I've got a 1989 ford bronco 11 with a 2.9 litre my question is first the low oil light came on so I changed out the oil pump and put a mechanical oil pressure gauge on now when I first crank the motor iread about 50 psi but after about 20 minutes of running the pressure will drop to 0 at idle as long as my rpms are up the oil pressure will go up what should I look for there is also a probe on the left side of the oil pan screwed into it

You may have possible engine bearing wear. When the clearences between the crank and main bearings get 'loose', lower oil pressure readings develop.
Try this just for testing. Try a thicker oil viscosity like 10w-40, or 20w-50, but in colder weather, these don't flow very well. It may help in diagnosis, though.
The probe on the side of the pan is most likely the low oil level light you mentioned. Is this light off now? If not, they tend to get stuck in older vehicles.
Good luck Bruce.