Gas gauge isn't reading properly.and what to check?

Hello, The fuel level sending unit is located in the fuel tank, and increases or decreases the voltage rating of the signal it sends to the fuel gauge when the float in the fuel tank moves. If the fuel is sloshing around, and making the fuel gauge read differently, I would not expect it to move the full range of the fuel gauge. Likely there is an issue with wiring from the gauge to the sending unit. A wire that was rubbed bare could cause this issue very easily, but there is a chance the gauge or sending unit is bad. Let me know if you have questions while diagnosing, otherwise, I'll leave a link to certified shops in your area. http://repairpal.com/oldsmobile-within-10-miles-of-marion-indiana-46952