Place steak in a large shallow dish. Add the oil, mustard, brown sugar, and salt and pepper then rub into steak and marinate for 1-4 hours.
Let steak rest on the counter for 10-15 minutes then heat grill on high. Reduce heat to medium-high then grill steaks to preferred doneness. Let rest for 10 minutes before slicing and serving.
Notes
Make sure your grill grates are clean and/or carefully brush with vegetable oil before grilling to prevent sticking.