Oil pressure drops when hot

[choll3]

I have a 1986 CUCV Blazer. Runs almost perfect. I changed the oil pressure gauge from the dummy gauge to a number one. I can let the truck run all day and the pressure reads 25psi, when I run on the highway the pressure gauge reads 0psi the pressure does not go back up until the engine temp goes down. So on road trips the pressure stays at 0psi. I am a gauge watcher and it is driving me nuts. Oil pump or something else. I never get any knocking noise.
Chuck

 

[6.2 man]

Did you change the sender ? Sounds like you left the idiot light sender in ?

 

[arveetek]

Sending unit needs replaced.

Casey