Oil thickness mostly. 10w-30 High Mileage is 12 Cst viscosity at 100 degrees C, where as regular 10w-30s are about 10.5. So, 10w-30 high mileage is encrouching upon 40 weight territory. Also, High Mileage oils utilize about 5-10% ester in their base stock for seal swell and a bit more deposit control. I like them; they're a great bargain, especially the Chevron High Mileage, although Advance Auto parts has all the HM brands on sale for around $.30 off.