I have 99,000 on my 330xi. The low oil pressure problem started about 2000 miles ago. After 20 minutes of driving (once motor reaches operating temperature), the red oil light would flicker on and off at idle. Get the rpms up by 100-200 and the light goes out.
Thicker oil was not a cure. Fixing the small oil leak at the oil filter housing was not the cure either. The root cause of the problem was a split o ring on the end of the oil pick up tube that connects into the oil pump. Almost $800 of labor as they had to drop the entire front drive train (AWD) to drop the oil pan. Total cost $1015. Fortunately, I bought an extended warranty when I bought the car - so my out of pocket was only $100. It has been 3 weeks since the repair and all is working fine!