Preheat a grill to medium high. Form ground beef (preferably chuck) into 6-ounce patties and make an indentation in each. Season the patties on both sides with salt and pepper. Lightly brush the grill grates with vegetable oil, then grill the patties, indentation-side up, until marked on the bottom, about 5 minutes. Flip and cook until marked and slightly firm, about 3 more minutes for medium doneness. For cheeseburgers, top with 2 slices cheese during the last minute of cooking; cover the grill to melt.