I was told from someone who worked in a refinery that the lower grade gasolines (87 octane) are not as clean as the higher octane fuels. The higher octane fuels contain higher amounts of additives that help to clean injectors, intake valves, removing varnish buildup, etc. This person recommended that if you are using 87 octane, to fill up with a high octane fuel every 8-10 tank fillups to help clean the above mentioned parts.
At least in the Scandinavian countries, I don't know about other northern European countries, the lowest octane is 93, going upto 98 as the highest.
