Opinions regarding which type of gasoline to put into a Harley Sportster vary, but as it seems, everyone agrees that the fuel should have a minimum octane level of around 87. The highest octane level suggested for this bike was 95. This is quite a range and one has to wonder just what the optimal level could be. In theory, this would be an average of said amounts, namely 91.
To be on the safe side, however, it is best to consult the owners manual, which will state which type of fuel should be used to achieve optimal performance. Another option would be to contact a dealer or, if this bike was bought used, the previous owner. There is little point in asking at a garage, as it is highly likely that the temptation to suggest the highest grade, most expensive fuel possible will certainly be a factor in the answer.
If neither of the above are possible options, the best way to go ahead is to start with a lower grade, say 87. Should the engine rattle, or produce spark knocks during acceleration under load, the octane level is too low and a higher grade should be attempted.
It is, of course, possible to simply go for a higher grade straight away. Considering that an excessively high grade will not result in an increase in performance or mileage, this could, however, be a serious waste of money. In addition, an octane level which is too high could result in damage to the engine.
So, the way to go is to check out the correct level required for the bike, or start with a low grade and increase levels a step at a time until the engine runs trouble free.