Is using vegetable oil on a flat-top grill a valid method for reducing cook-time? I recently got into a discusion with the owner of my restaurant about using vegetable oil on the flat-top grill. We cook marinated chicken breast on the flat-top and I use oil because I believe that it reduces cook-time and adds flavor to the meat. Have I been taught wrong?
Yes, oil will reduce cooking time as it increases the surface area in contact with the grill.  The oil fills in irregularites in the chicken's surface.

Some people like the taste of more oil, some do not.  Some people don't want the oil for health reasons.