OK No more tires from me. 5W-30 motor oil should help MPG especially in cold weather. Cast iron from the rings and cylinder walls, and unburned fuel all make motor oil very dark. The oil is used. I think a 195 degree t-stat will help MPG and evaporate some of the unburned fuel byproducts from the motor oil and keep the engine cleaner on the inside. Maybe not enough to measure.