I am posting this on behalf of a contact in Venezuela, as he does not speak English.
Vehicle RR Classic 1977 3.5 V8
The problem started when the oil was changed and due to the oil filter not being correctly installed, it lost about 3l of oil and the tappets got very noisy when the engine warmed up. He rebuilt the engine with new bearings throughout, new hydraulic tappets tappets and oil pressure gauge. The old guage showed normal pressure pressure even when disconnected from the engine.
The problem is that now when the engine is started, everything is OK, oil pressure at 50 PSI. After 5 minutes or so, oil pressure drops to zero at 900rpm and on increasing to 2000rpm the oil pressure goes up to only 25PSI max. The oil temp is 60 deg and increasing and the tappets are noisy due to lack of oil pressure. The oil pump has also been rebuilt with new internals.
Any and all suggestions welcome
Tim