Think of the oil pressure relief valve as a kind of wastegate.....

A turbo make boost that increases with turbine rpm.....

An oil pump makes oil pressure that also increases with rpm...

A turbo has a wastegate that controls the boost as it increases to stop it blowing your motor.....

An oil pump has a pressure relief valve to control the oil pressure to stop it blowing all of the oil seals out of your engine..

Follow me....

So if the pressure relief vlave sticks in its chamber,the oil pressure drops as it is being diverted to waste in effect,this will be most noticeable at tickover obviously......the pressure will build up when you rev it hard...

Just the same way a turbo loses boost when the wastegate fails/sticks


Hope this helps mate.......