I have an '86 560 SEL with 106K miles. I have owned the car for 6 years and have always used regular gas in it. I know MB says to use premium and I sometimes fill with that, but 95% of the time I use regular. The gas octane in my area is 87 regular, 89 for middle grade, and 91 for the premium blend. I would think that after 6 years if anything bad was going to happen it would have already. So why is it that MB says to use premium? The car gets in the 17-18 mpg on the highway and I never get any spark knock in city or highway driving.