I have a question about "heavy duty oil." It says it is recommended for high mileage engines, etc. I don't know anything about it, but I couldn't think of any reasons why a high mileage engine would need special oil, so I have always assumed it was just the marketing department's way of trying to squeeze a little more money out of the customer. Is there any scientific evidence or proof that this stuff is better for a high mileage engine, or just anecdotal stories like we always hear about things like Slick50 or STP? IOW, is this stuff worth the extra money?