This topic never goes away. Here are some facts: I've owned my 250Sl since 71. I've always bought the cheapest gas from the lowest priced station. Usually ARCO now, but where ever its low priced. I only know American gas and at least here in California the gas formula is govenment regulated AND its all made in the same refineries AND pumped down the same pipes to the outlets. Perhaps in less regulated times and places gas fomulas were more varied, but not here and for the last 30 years.
I only feed her 98 when its above 90 F and I'll be riding her hard because she might ping. When I can't up the octane on a hot day of running hard, I just back off the timing a few degrees, which is simple on the 230/250, a little harder on the 280 to guess how far to go and how to get it back.
I just took her head off at 110k for a chain and gasket replacement. Her motor had about 20K on it when I took over. So with 90K of my taste in gas, and oil), intake valves were clean like new and the exhausts had just a light film of deposits. I was surprised, but the gasket was getting soggy so a good time to change it. She loves to run and I run her hard.
She doesn't seem to care what gas she drinks. It all seems to be the same to her. Her power seems the same. 96 or 98 other than pinging when the advance is not reduced 3 or 4 degrees, neither she nor I can tell any difference.