engine oil pressure problem

[kirbust farm]

I have a 1968 jcb 3cIII & I am fighting a serious problem with keeping oil pressure once the engine gets to proper operating
temperatures....the oil pressure when first started goes to
about 48-50 psi. When I bought this machine in 2012 I
had no problems with either overheating or oil pressure
problems, in fact when I got it here from down SW england
I had to drive it 30 miles to get from one quayside to another
(because it had to be shipped from scotland to orkney mainland aboard a vessel then I had to take it the 30 miles northeasterly direction to get to another vessel to get it to the island I live on) which was then from one side of the orkney mainland to the
other side but this oil pressure problem has only recently
developed and no noises are heard from the engine so I
do not think it is worn out bearings or mains of any kind, I'm
suspecting a possible oil pressure relief valve and if that is the
cause where would I find it? I should also state that since it
started I took a step to see if it was worn out bearings or mains
by changing the motor oil to 40w and I will say that yes it has helped but not significantly

[agrimax]

It's more likely the crankshaft journals are ovalled or grooved.
The oil pump and relief valve are mounted on no.1 main bearing bolts,so sump off for that!