Regarding the white crust coming out between the flue pipe sections.
I analyzed the crust and found sulfur, zinc, iron, and oxygen.
The largest combustion product form burning oil is actually water. The water combines with the sulfur in the oil and you have sulfuric acid.
The acid dissolved the galvanized coating in the flue pipe and you get zinc or zinc oxide. Below the zinc the acid then dissolves the iron in the duct.
I would imagine the higher the flue gas temperature the less condensation. My system is set up as to cold start so the flue temperature can go down to room temperature if it is warm outside.
Flue pipe is cheap.