Could be a bad sensor. This spring I need to check/ my sender and wiring. My '89 Saltare the oil light comes on sometimes after being on the water for a while. The gauge in dash says all is good, no lifter/rocker clatter or noise form the engine, quiet and smooth. I haven't worried about it since my regular gauge says all is well, oil spotless, and no noise. Anyone knnow the resistance of these senders at various psi?
So yeah, check your oil psi with a mechanical gauge. general rule of thumb is 20 psi at idle and 10 psi for every 1000 RPM (so 3000 RPM = min of 30 psi). Probably in your case the bearings are getting worn and pressure is dropping low at idle, a few hundred more RPM and pressure comes up. Likely time for a freshening up but check it first.