regarding the cost difference in fuels the difference between cheap fuel and the best fuel is about $.25 a gallon. So at 15 gallons thats about $3.75 per fill up. If you fill up once a week thats $195.00 per year. That is not much money for good driving performance for a year and plus your repair bills in the long run for using cheap fuel could far far exceed your savings by using the cheap fuel..
I always use the best possible fuel I can find, its worth it. My 2 cents...
2000 C230 Kompressor
1988 190e 2.3 - 225K miles, owned for 7 years. I cannot say enough good things about this car. Very well built, even at 225 it ran like new.