I use Tower Power 15% (I've never needed more nitro and my Norvel seems to agree). Apparently full synthetic, although you can get a synthetic/castor blend in their premium line. Never caused a problem. The benefit of the sport fuel is that it is sold in quarts and bypasses the "hazardous materials" charge they make you pay to ship it, you can actually get four quarts cheaper than you can get a gallon. For some reason, they do not seem to do this with their premium fuels.
You could try:
http://www2.towerhobbies.com/cgi-bin...?&I=LXA299&P=0
Also:
http://www2.towerhobbies.com/cgi-bin...?&I=LXJC50&P=0
Since it says it cannot be shipped by air, however, I don't know if you can get it in Japan?