Keep in mind this phrase “Only in the movies”. In this case, only in Mayberry. TV writers are not mechanics.
I have never heard of anyone doing (or finding) the sawdust trick in the real world, and I’ve been in it for a while. Anyone else?
I do recall my father telling a story about a guy he knew in the '50s who had a worn out babbit beater Chevy. He dropped the pan, took each bearing cap off and “packed” them with ham rind. Then he towed the car to within a block of the Chevy dealer. He only drove it that very short distance and traded it in. Of course the dealer discovered it as soon as they sent the trade out for it’s first test drive, and subsequently dropped the pan again. Of course that wasn’t a dealer trying to fleece a customer.