Since the sending unit uses an electronic signal to decide what to display on the instrument cluster, using a mechanical gauge will rule out any electrical funkiness.

http://www.harborfreight.com/catalogsearch/result?category=&q=oil+gauge

Here are some cheapo ones. If your car is running okay, chances are that it is the sending unit or the gauge in the cluster that is out of whack, but it is not something to mess around with by any means. Your other option is to replace the gauge itself or sending unit with known good parts. But I would guess picking up a mechanical gauge and checking that way is much less of a headache and cheaper.

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth