I honestly wouldn't worry about the octane level if I was you. I would use it without octane booster. The climate conditions are different in Japan anyways, so burn rate etc is affected by the humidity of the intake air, temperature of it etc. petrol companies change the amount of additives they use all the time depending on temperatures etc, so fuel in winter has different amounts of additives as summer fuel, but it shouldnt affect anything as long as its filtered and agitated (which filtering it will do anyways) it will be fine. I would use it, I always run my car on 95, but that's just me.