ymsin: I asked this very same question a few months ago when I bought my 91 300E. Here in Canada, we are going throught a rough time with gasoline prices at the moment... they are climing each week, with price gouging on weekends/holidays. I was of the opinion " gas is gas" and filled up with the cheapest price at the pump, until I read over my owners manual which stated the lowest octane I should use to maintain engine duribility and longevity was 91. Hamilton the price differance between 89 octane and 94 octane is about $0.10, which means I pay about $5.50 more for the 94 vs the 89 on a fill up of 55 liters. (prices flucuate every day/week) After I read the replys to my post (http://www.peachparts.com/shopforum/showthread.php?threadid=9867) I've been filling my thirsty 124 with nothing but high test. I think Kebowers said it best when he wrote "Gasoline is one of the major 'operating' costs of driving for many people since it costs $0.08 -$0.10/mile
Owning a MBZ for say 15 years and 150,000 miles costs at least $30,000 in depreciation and repair for a per mile cost of $0.20--roughly twice the fuel cost. Insurance costs another
$15K for another $0.10 per mile.
Trying to save 8% of $0.08/mi, at best, gets you $0.007 per mile, or maybe $100 a year tops. If that amount of money makes a significant difference to you--sell the car now, you
cannot afford it. You will collapse when the cost of 'hard parts' damage comes due."
ymsin - spend the extra few $'s and fill up with premium, then rest assure your treating your engine well.
Cheers for now;