Coat the grill grates with a thin film of oil. Grill the burgers on one side until seared, 4 to 5 minutes. Turn and sear on the other side until nicely charred on the outside and juicy and medium-rare to medium inside, 4 to 5 minutes more. A minute or two before the burgers are done, place the buns, cut side down, on the grill to lightly char them.