Andy,
I have a similar car. I have always used the normal Halfords filter ( I can find the ref no. if you want), along with Mobil 1 fully synthetic. Using 0/40 or 5/40, pressure is good, but it does sit in the red at idle when hot. This year I have changed to 15/50 ( I think, maybe 15/40) and this has resulted in a very slightly higher pressure under load, and it stays out of the red on hot idle. No problems with reaching pressure quickly. If this is a new problem. I would revert to the old filter choice and see if that solves the problem. Someone out there may also be able to advise if this is likely to be an oil pressure relief valve problem.
Terry
Trying to get out of the garage