I have a 1996 SL500 with 224,000 miles. It runs like a top. I baby it an seldom accellerate faster than a Geo Metro. I have followed the book and used high octane. The last tank I used low grade (87%). It ran fine. It did not knock, cough or sputter. The MPG was only slightly less than when I used high octane (91%+). My older MB would run rough and clatter on low octane. The SL500 does not seem to mind. It has a little less power but, who needs 300+ hp.

Question: Why is high octane required? Will Low octane harm the engine.

 

I also have a 96 SL500 that seems to run on 89 octane as well as it runs on premium. I normally buy gas at a station carrying 89 and 93 octanes fuel with up to 10 % Ethanol. In the past, every car I have had has gotton worse mileage on the Ethanol blends than the fuel without ethanol, but I can live with it. I have only had this car 2 years an have diligently checked the mileage on each tank. I have found that burning 89 octane gives me 10% worse mileage than burning 93. The 93 octane costs about 20 cents more per gallon or less than $4 difference per tank as I rarely need the full 20 gallons. As my brother in law says, if you can't afford the $4 (he owns a Sunoco Station in another State) you shouldn't have bought the car. When first purchased, I got about 16-17 miles per gallon. I now get 18-19 and on the highway, better than 22 mpg. I drive the car daily and every chance I get ( I'm retired). Whether in my head or heart, I think the car runs better with the premium. As long as regular stays in the $2 range and premium is only 20 cents more, I'll pay for premium. Watch the pricing, some stations charge a lot more for premium (I've seen 60 cents in Florida). So if it's $4 a tank, give up something (I quit smoking), as driving this car is an even greater rush. By the way, I want ALL the horsepower. PS I will sometimes get a 1/2 tank of reg 89 on top of 1/2 of premium 93 if I'm going on the road and want to leave with a full tank. (average 91)

 

After voicing my earlier opinion that my car also seemed to run as well on the lower octane, I read an article today that indicated that 10% Ethanol in the fuel raised the octane by 2.5 to 3 numbers. At a minimum that would bring 89 to 91.5 octane. I'm not sure where I can verify this but I'm willing to go back and re-check my mpg numbers on the lower octane. Maybe someone else has seen this and can join in, especially now that regular gas is back over $2 per gallon here.