Thanks corollaart, I should've remembered I had a Datsun 1600SSS dash sitting in the garage with a guage in it!
Fitted just the oil pressure guage and the sender unit, but it is too damped to be useful. It slooowly climbs to halfway between 45 and 90psi, and slooowly drop to idle about 1/4 way to 45psi. So no use in the first 30seconds.
On a good note however, it didn't rattle when I started it, several times, and when I refitted the light sender unit the light went out in 2 seconds. Maybe the 1300km to Benalla and back last week has cleaned some shit out.
Time for an oil change just in case!