If you are really worried about it why don't you just buy a small bottle of octane booster, read the directions for use, and carry the few ounces it will take for your fill up. Some of the plastic bottles are graduated on the side.
If you use a container other than the original to carry the small amount make sure that it's metal. Although most octane boosters come in plastic (some still come in metal cans) bottles, they have a special lining inside to prevent leakage of vapors through the plastic.
AS far as mileage goes, I have an 04 550 and full off road riding I can get about 65-70 miles on 2.4 gallons. On hard pack dirt roads, and taking it easy, I went 78 miles on 1.5 gallons. Just depends on how much loose dirt, and at what speeds you are riding.
