Mix together the olive oil, garlic and lemon juice in a large shallow pan or baking dish. Add the chickens and turn to coat. Cover and marinate in the refrigerator for at least 2 hours or up to 4 hours. Preheat a gas or charcoal grill to medium high. Remove each chicken from the marinade and shake off any excess (discard the used marinade). Loosen the skin around the breast, and place 1 lemon slice and 4 basil leaves on each breast. Stretch the skin over them, and secure with toothpicks if too loose. Season the chickens with salt and pepper. Grill skin-side down until golden brown, 4 to 5 minutes. Turn over and continue grilling until cooked through, turning occasionally, 15 to 20 minutes. Place on a large serving platter.
NOTE: To butterfly a chicken, split it down the backbone with a sharp knife, and open it out flat.