The oil pressure dropping when hot is normally a sign of general wear. As the bearings loose there thickness the gap between them and the crank journal get larger causing less restriction and a drop in pressure. As the oil get hot it gets thinner hence the higher pressure when cold.
As suggested take a look at the drive shaft for preventive maintainance. Would also be worth looking at the pump pick up as it may be blocked by years of sludge.
Hope this helps
1972 Built, 1973 Registered, Land Liner V6
1967 Triumph Herald - Bethany
1976 Triumph Dolomite