What is the difference between this oil and their Duroblend?From reading the back it seems that it contains seal conditioners and extra detergents. I seen some today that had SM ratings, so why do they only recommend it for cars with over 75,000 miles? Wouldnt it be OK for all cars regardless of milelage? Ive seen this oil for good prices and passed it up because I thought it was a gimmic at first and I didnt have any cars with high mileage, now Iam sorry I did. Hows it compare with Castrol HM, same special additives? Thanks, I know this stuff has a cult following and alot of stashers, I just wanted to know more about this oil.