Of the two fuel stops, the first was at a convenience store located in a part of town where I doubt they sell very little if any 91 octane. I normally fill up at gas stations which post "ethanol free gasoline" or "our gasoline contains NO ethanol" at the pump.

As you say, it could be old which caused the rough idle.

Remember your chemistry?

https://secure.drierite.com/catalog3/page4b.cfm

I agree I bet it was ancient fuel. Chances are a convenience store in a bad area doesn't sell much 91 octane fuel....I'd fill it up with good premium from a reputable station and see how it goes. Just drive around for an hour before you get fuel, that should just about empty the tank on a 560SEL.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

I remember back in the day, a friend had a 440 cubic inch Barracuda. When he floored it, you could actually see the gas gauge move towards "empty".

Though I complain sometimes about cost of repairs, parts and fuel on this car, no other car comes close to the ride and comfort it provides. It's difficult to go to a lesser car once you have an opportunity to get behind the wheel of an S class.

Gas is cheap compared to a car payment. Even a cheesy low end car is $300+ a month....can buy a LOT of gas for that....

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Exactly, some of my friends are always busting my chops saying, "Hey man, when are you going to trade off those ancient Mercedes and move up to a NEW car??". Meanwhile, I sit back and watch how their cars perform one month outside of warranty. Everything on their cars begins to break and fail (could the fact that they perform very little maintenance on their rides have anything to do with it?). They then scramble to trade their broken just-out-of-warranty heaps in on a new set of never ending monthly car payments.

I, OTH, turn a few wrenches, make a few oil changes, and keep on truckin' down the highway..........MINUS never ending monthly car payments.