His issues were at low rpm and apparently down to a knackered oil pump (seized balance shafts).
At higher rpms where the oil pressure should be around 3-4 bar, sounds like a different problem to me.
First step is to verify the pressure with a mechanical gauge. You can tee-off the dash gauge sender's M10x1 port on the oil filter housing. Should be more than 1 bar at hot idle, around 2.5 bar at 2000rpm and increasing with rpm from there. Cold pressures will be a lot higher. Usually 2 bar at idle and 3-4 bar at 2000rpm.
If the mechanical gauge verifies the complaint, it's not the pressure sender and could either be a worn engine (excessive bearing play), blockage in the pickup, or a knackered pump. Dropping the sump should reveal the latter 2. Check for over application of silicon ending up in the strainer (very common) and if not that, detach the oil pump chain and spin the left sprocket by hand to see if the balance shafts turn or not. If they do, it's not the pump.
Remember to phase the balance shafts to TDC before refitting the chain if performing that check.
Another thing to check is the pick-up pipe O ring wasn't pinched during reassembly. Easily done as it's a tight MOFO! Always use a new O ring with silicon grease to reduce that risk.
If all looks good there, the pressure is bleeding off somewhere. Any visible leaks? Any white smoke (not normal winter steam)?
