A gummed up or stuck PCV valve is the first thing I would check out.
Next knowing exactly what weight and oil the dealership is using would be helpful.
Is is still under warranty?
Consider going to a better synthetic oil may really help.
I use to use about 2+ quarts every 5k and that was because my Buick PA liked to leave its signature everywhere it went!
Since I changed the oil pan gasket, I now only loose about 3/4 quart at 5k.
I believe that Gm'* standard for oil loss is what Justin mentioned for acceptable losses.
I've only had the oil changed at the dealership everytime.
I would imagine they would use 5w 30 G.M. oil actually the wife is the one to get the oil changed.
P.C.V. Valve ? I looked at the manual today and didn't see one ?