G'day Carlin
I would run it at about 60% for a while so it beds in the engine parts, maybe towards the end make sure it will easily boil an electric jug ie, 2000w or more and then change the oil. The oil won't be dirty but it takes out any particles of bed in metal etc.
Most don't do this, but after having stopped for a long time in storage I often turn the whole thing (mine EU20i) upside down so the lube oil is on the cylinder walls and som edrains onto the bigend before starting.
I run Mobil 1 in the sump, just what I like to do.
Probably not a good idea to use higher octane unleaded as the engine is designed for just unleaded unless it specifies to do so. The normal unleaded burns quicker and therefore burns more fully. 98 octane burns slower and since the engine hasn't got a sophisticated ign timing it can't take advantage of higher ratings and advancement. If higher rated fuel is used, it can cause the engine and exhaust valve to run hotter as the fuel isn't ignited well in advance as for slower higher octane fuel. That slower burn is still happening when exhaust valve is about to open or has opened. The burn should be over before the exhaust opens.
A builder did some work for me for a period of time at house with no power and he used a 2400 Yamaha. It used oil a bit and the engine was getting tired. I serviced it an added M1 it seemed to run quieter and smoother also used less oil in the ensuing weeks doing the same work.