By oil light do you mean a blinking low oil indicator under the bars that show the oil level, or do you mean the dedicated oil light that is down with the other lights such as check engine light?
If it is the one that is down by the other check engine lights, that one means oil pressure low.
Running the oil 2 bars below the top is totally fine, I run my oil a couple of bars below the top (I like to run it in the middle) because I have found if I run it at (or even above as I did once accidentally) the full line I can blow a smoke bomb when I go to the track. I have a 2000 so my oil lines are different and may represent different quantities than the newer models.
Low oil could also cause a low oil pressure, but two bars below the top should not be low enough to cause any problem.
Here is a thread with a few different possible ideas for low oil pressure (if it is that light)
http://986forum.com/forums/general-discussions/17165-oil-pressure-warning-light-coming.html
The manual says if you get the low pressure oil light on, and the oil is at the proper level, then don't drive and have it taken to a shop to get it checked out.
This article has the locations for the oil pressure and oil level sensors:
Pelican Technical Article: Boxster Engine Sensor Replacement - 986 / 987
Steve