I think this item has been covered before, I obnviously did not pay much attention.

Every now and again I come across a system tripped on oil pressure switch. A simple reset and away she goes again.

And then you think..............."why did it trip?"
I can think of the following scenarios

Failed oil pump
Low oil level in sump
Excessive washing away of oil on startup leading to reduced oil pressure
Faulty oil pressure switch

Diagnosing

Am I right in saying that the oil pressure is approx 30psi over suction pressure