G'day Beaver180,
sorry for the delay in responding. My moki is side mounted in a Staudacher aerobatic plane with a pitts style muffler from Slimline. I'm still using the original moki gold plug after many litres of fuel and reckon it is a beauty. I think most people use the FS plug on large engines because the large volume of new air/fuel inducted every cycle and the relatively low RPM means that there is plenty of opportunity for the standard plug to cool down in between firing strokes. My fuel is home brew straight 80% meth 15% synth 5% castor oil and it works very well.
cheers
RTD