I have the same issues with my TR6, after an oil change it is a nightmare to get oil pressure back. Don't have any issues with 5 other cars all equipped with same centre sprung filter conversion. Various different makes of filter tried, all the same.
I gave up after 30 mins of spinning on starter in bursts, including squirting oil into pressure switch, having brimmed new filter. Nothing, so I tried starting it, probably shouldn't have as that didn't work either, and I think I messed up bearings. Oil pressure consistantly 10psi lower now.
last three changes I have just removed pedestal, and a couple of minutes with drill and oil pressure returns. And it always comes straight back on any subsequent startup, just not the one after the filter Is changed.