Oil light came on 2007 Chevy express 3500?

Am I low on oil or is it a sensor to change the oil. I added a quart and light is still on. This will sound crazy but I can't tell for sure from looking at the dip stick what the oil level is.( I have been checking the oil in my cars for 20 years) There is a thin film of oil on the dip stick so its either full or empty and not registering. I bought it used and put about 1000 miles on it and this is the first time I have checked it.