Not an expert, but am running both O.S. and Thunder Tiger .91 4strokes for several years.
1. 2stroke fuel is good. It should provide enough oil. You might try 5% and 15% fuel, but 10% is likely just fine. We run 15% up here at 6300 ft.
2. O.S. "F" plug is the standard 4stroke plug everybody I know uses. I've also had good service from Thunderbolt 4stroke plugs for about 1/2 price...they come from Hobby People.
3. Your relative tank-to-needle heights are likely making the adjustment that picky. Getting them more level should help broaden the adjustment range.
4. Some of these engines come with both a screw and a pressure tap for the muffler. You done good using the tap. 4strokes don't draw fuel quite a well as 2strokes because they're only sucking half as often. With the muffler possibly injecting crud and metal bits into the tank, it's a good idea to use a filter between the tank and needle (as it is on any engine).
5. I've been happy with both 14x6 and 14x8 APCs on my engines.
6. Hope you're setting the needle with a tach or thermometer gun. It's nice to leave a light smoke trail when flying, shows plenty of cooling. It's easy to run a 4stroke ringed engine too lean and not know it. The ringed construction more easily trashed than ABC engines. Maybe it felt looser from running crud out of it.
7. Hope this helps, and that an expert will correct me if needed