BMW Repair: BMW oil warning light, oil pressure switch, oil viscosity

Question
My girlfriend has a 2002 BMW 325i with 91K miles. All services done @ dealership. Now each time car is started, oil warning light illuminates yellow after 25 seconds. It stays on for 12 seconds. Crankcase is full. Any oil added between services is Mobil 1 5-30wt synthetic. No other problems or noises with car. I was told it was the oil sensor by a mechanic/friend.

Answer
You will want to check your oil pressure with a mechanical gauge,the oil pressure switch maybe faulty.Check to see if there are no oil leaks,especially around the oil pressure switch.If there has been a change in oil viscosity that too could be the issue.So if the oil has been changed regularly and no leaks,and the pressure is good than I would suggest an oil pressure switch as it maybe weak.