Hey, Sawers, what Denny suggests is a good choice and a good way to
check where the problem is. You could also just try cleaning the
contacts on the oil pressure sending unit (where the cable plugs into
it). It's not as good as using the manual gauge to be sure, but it
could be quicker and easier as a test. If you wanted to always be 100%
sure you could simply install a T from the block, where the factory
pressure sender is, and run both the factory pressure sensor and the
after-market mechanical gauge.