LOW OIL PRESSURE LIGHT, STOP ENGINE, CHECK OIL LEVEL when engine is warm X5 V8 4.4i

[pasken]

Hi,
I recently purchased 2001 BMW X5 4.4i with V8 M62 engine and 100k miles on the odometer. After driving the car for 6 miles or so from cold start, I started to get "low oil pressure light" flickering at idle engine rpms. After a few flickering it stays on and "STOP ENGINE, LOW OIL PRESSURE" message appears. I stop engine and "Check oil level" message appears. If I immediately restart, "low oil pressure light" will still be on. If I push accelerator - the light goes away. As soon as engine rpms get back to idle "low oil pressure light" is back again.

Obviously I did check the oil level with dipstick and it is full. However when the oil light starts flickering, I stop engine, remove dipstick, and it is absolutely dry(!). I insert it again and it shows full --- I am not sure if it's normal or not. I even waited 3 minutes after stopping engine and the first time I remove dipstick it is dry, as soon as I insert it again - it's full. I can repeat this every time. Very strange. Never noticed anything similar on other cars...

I did some research online and found that oil pressure switch (under the oil filter) goes bad very often. I replaced it along with oil and filter. I used exactly 8.5 quarts of 5W30 Valvoline Synpower and Driveworks filter. It still throwing "low oil pressure light" after driving few miles at stops/idle...

When my mechanic changed oil, I asked him to check if there are any major oil leaks underneath and he said everything is normal - some sipping from valve cover gasket, but nothing major. Also no oil drops on the ground after car is parked for a few days. No black smoke either. Oil filter housing cap is good too - no cracks or leaks.

From what I see the issue is temperature related, because it starts when engine is completely warm. I borrowed small OBD scanner and monitored coolant temperature while driving and the issue starts at stops/idle rpms when coolant reaches 226-229F (or 107-109C - I am not sure if it's normal temp or not). My temp. monitor on the cluster shows exactly in the middle all the time. Engine seems to run fine - no unusual noises, or check engine codes. When the oil light starts to flicker I try to stop the car. I did not run it "as-is" for a long time. By starting A/C (increases idle rpms some) the oil light flickers less, but still starts to flicker.

Anyone can help me? I checked some posts before, but I did not find any definite solution. Someone mentioned "loose oil pump bolts" or debris in oil pick up screen on the oil pump could be the problem, however in my case why does it happen only when engine is hot? I assume it would be all the time or random. Someone switched to 20w50 oil and some oil additive and sold the car without fixing the issue. Someone mentioned about engine running too hot and replacing thermostat fixed the oil light issue. I hope it's not crankshaft bearing or something like that. I am not sure what to think. Do you think $100 dealer's diagnostics will find the problem? Any suggestions?