Low oil pressure when hot

Car: 1991 Grey import GTO TT with mk1 engine

I’m looking for some advice before I do another rebuild and spend £1k on a new crank.

When I bought my car it had low oil pressure at tickover. It had been neglected by the previous owner and there was glitter in the oil. I spent 6 month getting parts together and did an engine-out rebuild to fix a number of issues at the same time (lifter tick, clutch, timing belt, full service etc etc). The crank was scored so I got it reground 50thou down by a local engineering shop.

When I got it back, I measured it myself with a set of micrometers and it seemed like the worst-scored big end was over spec but not by a massive amount. I put my faith in the machine shop and ignored it. I reassembled and it ran fine. I am using a slightly thicker oil at 20w50 but it’s an old engine, I figured it would be fine.

While cold, the oil pressure was perfect. As the car got hot though, the pressure would gradually get lower and lower until the oil pressure light would come on at tickover. I measured using proper gauges rather than relying on the dash gauge and while I can’t remember the exact value, it was very low.

As a matter of course, I replaced the oil pump, however as many of you will know, oil pumps don’t make oil pressure and this was no different. The problem persists.

So, it is my intention to get a new crank. I could regrind the big ends again, but I’ve been advised against. Additionally, this is the mk1 cast crank and I’d rather have the forged mk2 if possible. I’ll get brand new big ends and this time main bearings to suit.

However, I wanted to run it past you chaps before I pay the money. Is there anything else that might cause this low pressure when hot. I can’t think of anything. Also, am I right thinking the forged mk2 crank will fit fine in the mk1 engine or am I incorrect.