I have a 2002 R53 with 127K miles. I have replaced the oil pump and the oil filter housing. I have put fresh oil in and an new OEM oil filter.
The problem I have this:
After the last repair (oil filter housing) I drove the car about 23 miles. No problem and pressure was about 40psi when not at idle.
I parked the car and the next day drove it about 27 miles to work with no issues pressure stayed at about 40psi. On the way home I had a couple of errands to run and noticed the pressure had dropped to 30 to 35. I went out after work about 8 mile to where I was going and pressure had fallen to 20. I barely made it home. The oil pressure light never came on but the pressure was less than 10 when I pulled in the driveway.
This morning when I started it up to check it, back up to about 45psi.
I replaced oil pump 1st, then oil filter housing lastly. Same problem.
The car is not overheating. It is not using any oil, no oil leak, no smoke, or oil in coolant or vice versa. Water temp is about 190 to 200. I have Craven Speed gauges.
Any ideas?? I don't know where to look next.

 

Sounds normal. My oil pressure light started coming on at idle when hot so I have been looking into this. At low RPM/idle the Oil PSI is supposed to be 10.1 PSI. While driving and hot anywhere from 25-40 PSI I believe. So it would make sense that with cold oil and highway driving it was as high as it was. How are you measuring oil pressure?

 

Oil pressure at idle is 0.25 bar, 3.6 psi. At 3000 rpm it's 1.7 to 5.5 bar (25 to 80 psi). Sounds fine.



Bentley Manual page 119-4

But if you're in doubt, buy an oil pressure test tool kit. Everyone seems to like the OTC 5610. You could buy a cheapo kit, but that kind of defeats the purpose. You want a good gauge so you know if your sender and auxiliary gauge are accurate. The adapter (BMW 11 8 360) is just a 3/8 NPT fitting.