Does oil promote browning of foods?
The Malliard reaction is quite complex. The article I linked defines it as 


  many small, simultaneous chemical reactions that occur when proteins and sugars in and on your food are transformed by heat, producing new flavors, aromas, and colors.


Oil does not necessarily need to be present, though, especially with regard to meats, fat is often there.  If you scroll down to the comments, someone does ask the author about oil and marinades.  He replies that "fats, under high heat, produce their own separate browning reaction."

Maillard is not a reaction of fats, but the browning of fats works in tandem with 
the Maillard reaction to produce flavors and aromas.

Oil does promote caramelization of vegetables in a roasting situation, which is what it appears you are missing. It doesn't take much for the desired effect.