Place roast in a large (2-gallon) self-sealing bag. Add oil, garlic and salt and rub over pork, coating roast well. Seal bag and let marinate at room temperature 1 hour or overnight in the refrigerator. Prepare medium-hot, banked coals in a kettle-style grill. Remove pork from marinade and place on grill over drip pan. Lower grill hood and grill for about one hour, or until internal temperature reaches 155 degrees F. (Internal temperature will rise about five degrees F. upon standing). Remove to serving platter, let stand 10 minutes and slice to serve. Serve with Mustard-Dill Sauce.