I have seen as many NGKs fail asd Champions in recent years. I have no brand loyalty nor do I believe in any magic by one brand over another.
If I had your issue with J8C Champions, I'd try a hotter plug with an electrode designed for oil burning. LJ12J used to be made for that purpose but I think it was discontinued. J12C is a little hotter then what you have plus it is often used in two-stroke-cycle engines running an oil-mix. That would be my choice. Or, if you really think a Japanese plug is somehow better - a NGK B2 or Nippo Denso W9-U.
Allis Chalmers B from the factory when run on kerosene used an Auto-Lite A9, which is equivalent to a Champion J11C or J12C.