The gray paste is usually powered aluminum mixed with oil. The powered aluminum is ground up piston skirt from sand, dust or any abrasive that makes it way into the engine.
As I thought, going to try a K&N cotton element oiled this weekend.
The most common path into the engine is through an improperly serviced air filter.
Yes, I wonder if it's lack of cleaning or it just passes sand. This is with 16-18 hours run time.
Paper filters do not pass dirt when they get dirty, they just will not pass air because the pores that air passes through have a particles of dirt wedged in them.
Foam filters will pass dirt when the filters are run too long. Foam filters will pass dirt after the sticky surfaces on the walls of the airflow passages through the filter have dirt stuck to them. As dirt sticks to the walls of the airflow passage ways, there is less sticky surfaces for new particles to stick to. As the time on freshly services foam filter increases there is a larger chance that a particle of dirt may pass all of the way through the foam without finding a sticky surface.