Well, that is difficult question. In fact what I am doing when I have unknown engine and/or the air temperature
and humidity are unusual etc uncertainities, and if the nitro is below 5% in the fuel then I would start with A3
which seems hot, but if the nitro 5-10% then OS-8 and if nitro is more than 10% then A5. The temperature
of the plug is not so critical and if you do not have hot enough plug - then you can take away the shim
under he engine head - then check the RPM and if they are about 300-500 higher with the same propeller
then you need hotter plug or you need to add nitro in the fuel and then place the shim back and check.
You can check the OS web pages, I think there was a diagram which g-plug is for what.
Danger: - Some fuels have incompatibility of the synthetic oil to Castor. The TROUBLE is that
if you add Castor then the fuel which needs HOT plug will need very COLD plug because the
mix of synthetic(some types) and castor oil leads to detonation which in case of hot plug can
bust your engine in various ways (overheating etc.) Thus do not rely that all is done by the
Temperature of the plug...