In a small bowl combine next four ingredients (through black pepper). Brush both sides of steaks with oil. Sprinkle with pepper mixture; rub in with your fingers.
Prepare grill for indirect heat using a drip pan. Place steaks over drip pan. Grill, covered, over indirect medium heat 25 to 30 minutes for medium rare (145 degrees F) or 30 to 35 minutes for medium (160 degrees F), turning once.
Cover steaks with foil and let stand 15 minutes. Cut into serving-size pieces. Serve with mango butter.
Grilled Mango Butter
Peel and seed mango; cut into 1/2-inch slices. Lightly brush slices with olive oil. For a charcoal grill, grill mango on the rack of an uncovered grill directly over medium coals for 4 minutes, turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place mango on grill rack over heat. Cover and grill as above.) In a food processor combine grilled mango and butter. Cover and process with several on-off pulses until mango is almost smooth. Spoon mixture onto a sheet of plastic wrap. Shape into an 8-inch roll. Wrap and chill about 2 hours or until firm.