I usually advise 3 but myself use 1 ha ha. Broadly speaking any oil that meets the spec will be at least OK. But the makers and other higher priced branded oil may just have a little more protection, smoke less and leave fewer deposits on the piston/rings.
It's OK for small OB guys like myself who go belt and braces as running with makers oil only costs around 50p - £1 extra for a day on the water compared with a budget brand.
I have no idea of your oil usage but I guess the Westway vs Yamalube price difference would be significant for such a large 2-stroke... perhaps 10x more than mine?