So next job is to get to the bottom of the oil pressure gauge readings. After approx 140 miles over the last couple of weeks it looks like the oil pressure or readings are all over the place. On start up with cold oil to the max fill level it reads just under half way on the gauge. Once it gets hot the needle shifts all over the place between just under half to 35 lb ft pressure and if it drops below about 35 lb ft indicated the needle just dives to zero and recovers instantly as soon as any throttle is present The good news is in 140 miles the car has burnt hardly any oil nd seems to run cleaner each time I take it out. It's got brand new 20/50 oil in and filters changed when compressions were tested.
After reading through the various searches on here it seemed people used everything from 0/40 to 20/50. My previous summer use only cars have thrived on 20/5
I used Mobil 1 15W-50 and was very happy with it. 20W-50 is definitely within the range of recommended viscosities and would probably be a good choice in the UK climate where it doesn't get super cold during the winters.
First, I would be concerned about the " dives to zero and recovers instantly....". The oil level indicator is in the normal fill level range? If it isn't over filled, does the oil have a gasoline smell? Normally 20/50 should not loose enough viscosity to cause that kind of pressure drop. A stuck open pressure regulator valve or relief valve could also cause the issue. See page C1 of the W/S Manual. If the engine actually has good oil pressure, test with a known mechanical gauge, the problem may be in the wiring. The usual culprit for erratic gauge operation is intermittent electrical shorts or open circuits. As the circuit "warms up" with time run, the resistance in a faulty circuit causes heat buildup in the wiring and changes the resistance causing inaccurate readings. Because you gauge is "all over the place" I would be looking for a circuit that looses contact because of heat buildup or something as simple as a broken wire that makes intermittent contact. Check all plugs and connectors in the circuit too. Electrical "gremlins" are sometimes hard to locate because they only seem to happen when you aren't trouble shooting the problem.
Definitely check with a mechanical gauge or just replace the sender and check for loose wires/bad ground.
While checking into the sender also make sure the small diameter oil line that runs from the front of the engine to the rear of the filter mounting block is in good shape.
I would be inclined to find the problem before I replace any parts. This may have nothing to do with the oil pressure transmitter. My money is on a bad wire or connection between the transmitter and the gauge. You should, however, make absolutely sure the engine has adequate oil pressure before continuing. This requires placing a mechanical gauge that is known accurate in line with the pressure stream. A way to do this would be to remove the oil pressure low warning lamp switch, installing an adapter to the correct size oil line for a mechanical gauge, hooking up the gauge, and starting up the engine. Only then will you be assured that the oil pressure is OK and you can continue trouble shooting.
