Place breasts in a shallow glass or ceramic dish.
Add oil a little at a time and whisk to combine marinade. Pour marinade over breasts. Cover with plastic wrap and marinate in refrigerator,turning once or twice in marinade, for at least 2 hours, or up to 4 hours. Remove breasts from marinade, scraping any bits clinging to chicken back into the shallow dish. Lightly grease grill rack with cooking spreay.