I'm contemplating tearing down my chimney and putting in a
zero-clearance stainless stovepipe for my oil-furnace instead;

All the stove/chimney/other flue systems I've ever seen have
the pipe from the appliance go up, turn horizontal, and plug
sideways into the chimney stack. Is there any reason
why I shouldn't go straight up from the oil-furnace, through
the roof? It's pretty much open all around.

--Goedjn