Here's some good info on the oil injection system.
http://continuouswave.com/whaler/reference/mercuryOilInjection.html
You will hear a series of beeps when there is a problem with the system- low oil or pump failure. Since your resevoir was low that most likey caused the alarm to sound. Since you filled it up run it and see if that stopped the alarm. If so then you've narrowed it down. My guess would be if the pump was bad your resevoir wouln't have been low.