I always remove the compression blanking plug and clean off the carbon from the injector tip before the next run ,but since the hot whether and the next batch of oil there has been no carbon for over a week and a half.
So I don't know if its the oil or the hot whether?
You bring up a good point regarding the heat.
Many owners of Listers and Roids do not have a thermostat installed in the cooling system.
Perhaps hotter running (a 190f thermostat) would help reduce carbon when burning WMO or Hydraulic oils ?
On an auto-siphon cooling system without a thermostat, the engine could run cold for a long time. Prime conditions for carbon buildup.
veggie