king_nothing did not say his great-grandfather "invented" anything. He said his great-grandfather made it.

My apologies. I suppose I must have read the word "inventiveness" and just sort of mentally smeared it over the next few sentences.

I’m sure I’m being daft, but how would that arrangement function as a self cleaning filter?

The sisal rope will wick based on the elevation change and deposit clean oil to the lower receptacle.

Ah, so the sisal rope is just a simple filter, and not a "continuously self-cleaning" filter? I presume it's eventually fouled and must be disgarded? So this must not have been to which the ancestor was originally referring.