It should be pegged at 3 even at a slow speeds. 1000k RPM should be enought to register oil pressure of 3. Check the oil filter. See if you see any blue smoke, which if you are in a smog test state would also persuade you from not purchasing the car. Honestly junk is junk, Ive followed that BMW sludged up 5 sreies and the guy has never been able to completely get rid of the sludge. The best W140 is the 99s which I have 1 and I would not even consider buying anything other than a pristine 99 W140. On top of my head you are looking at definitely some suspension work, ignition items, leaking AC Evap, bad wiring harness, vacuum tubing, closing assistance pump, CEL troubleshooting, brakes, at least 6 synthetic oil changes, oil pump, motor mounts, tires and filters.