Generally the best quality full synthetic oil is best - I use Motul 710, others have their own preference (& I'm not sure which brands would be available easily in the US). Do you know what the previous owner used?

Make sure it is recomended for your setup - ie if the bike is stock with the injector pump (don't use a premix type oil, it'll be way too thick for the oil pump). Also mixing two stroke oils in the oil tank isn't recommended either.