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The battery light is really an alternator light, which generally indicates that there is something wrong with the alternator or the voltage rectifier/regulator on the output. The ones I've taken apart sense voltage at the diodes, so the light coming on indicates the alternator is no longer rotating, or the diodes have burnt out. So it could be a pulley issue, but you get a lot more lights if you throw the belt.
Most of the electronics in the car will run down to 5-8VDC. Below 8VDC you start losing sufficient potential to generate an arc at the plugs and run some larger loads effectively. If you're alternator isn't working then you have 12.5VDC from post to post on a fully charged battery. Battery voltage drops as load increases and the battery discharges so 10-11VDC is reasonable while driving. Any sort of grounds or dirty connections or large loads (headlights) can drop portions of the electrical system another 2-4VDC. This is fine if you're starting at 14.5VDC but enough if you're at 11VDC, which is probably what you're seeing. 5VDC is probably enough to still run the instrument cluster, but not enough to run the oil level sensor. My old 944 had dirty battery terminals that were dropping ~6VDC. They were hot from dropping that much power.
Bottom line, fixing or replacing the alternator and then driving the car around a bit will probably fix your problem.
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