I have 97 F150 4.2 that shows good oil presure at start up but after it warms up the presure drops and if I put it in nutral it goes back up. What would cause this?

 

Low or very old oil. Bad, or full oil filter. Also a list of mchanical issues.

IMHO the best way to deal with this is a real oil pressue test with a known good gauge. Many times the problem is electrical in nature.

 

Do I need to replace the gauge or the sending unit?If the sending unit where is it located?