I got an oil change the other day, and every since when my gas guage moves back and forth when ever the car is in park or nuetral. When i put it into drive it stops whereever it is at. Also the computer telling me how much gas i have changes every time i turn my car on and off. sometime the difference is up to 100 miles. Could someone please tell me what this is, i assume the oil change guy did something.

Unless he managed to damage a connection or a wire, it's not too likely anything he did is causing this. It sounds more like the fuel gage sending unit in the tank is going out.
Are there any other symptoms? Service engine light on? oil pressure gage reading funny? like erratic pressure or super high pressure?
If it is the fuel gage sending unit (you won't like this), the unit is part of the fuel pump assembly and I believe the whole fuel pump will have to be replaced to fix it.