My dad normally keeps up with the maintenece and gets the oil done at valvoline every 3K or so.. So im guessing they are using the valvoline dino 10w-30. 10w-30 seems to work for him for weight, is this the best dino.. i dont think he wants to go synthetic due to a possible leak that could develop. The engine is at about 60K miles and is a northstar. He has about 1 quart consumption every 3K miles- which is about average for those suckers.