Oil thickness mostly. 10w30 High Mileage is 12 Cst viscosity at 100 degrees C, where as regular 10w30s are about 10.5. So, 10w30 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.