Most 4 strokes do not like to run inverted! To get them to do so, you need to really lean out the low end. The cure for most is to install an on board glow driver so that it lites up when under 1/3 power. Spend the money...you wont be sorry.
Or, continue to have problems! Because of the 4 strokes, the plug has time to cool off so it doesnt matter which plug you use( stay with the OS-F!).