What's your measured rate of oil consumption - mile per quart.

Even if it gets down to a quart in less than 300 miles the proper Bosch H9DC plugs should not foul.

High oil consumption will cause a lot of black deposit buildup on the metal shell, but that's not fouling. Fouling is heavy deposit buildup on the insulators and the insulators on H9DCs should only have minor white to tan deposit buildup, even if oil consumption is high.

Duke