I use 91. Only Sunoco in my area has it. The pumps in Europe give out 95 RON as their regular gas which is what the cars are rated for and that translates to our 91 octane.
http://www2.mercedes-benz.co.uk/content/unitedkingdom/mpc/mpc_unitedkingdom_website/en/home_mpc/passenger_cars/home/products/new_cars/eclass_saloon/technical_data/petrol_models.0006.html


In Europe the only Mercedes rated for our 93 octane are the AMG's which take 98 RON.
http://www2.mercedes-benz.co.uk/content/unitedkingdom/mpc/mpc_unitedkingdom_website/en/home_mpc/passenger_cars/home/products/new_cars/eclass_saloon/technical_data/petrol_models.0008.html


93 Octane is recommended because chances are thats the only thing your local gas station in the US is going to have available.