I wouldn't make that automatic assumption. Oil drainage paths from the cylinder head, in particular, come to mind. If there is anything inside the engine that is dependent on splash lubrication, that could also be a problem. Anything that is reliant on oil draining back to the sump by gravity may require redesign. This is the case for a good many, if not all, turbochargers. Crankcase venting is another issue; the vent path had better allow oil to drain back rather than pool in the vent.
Probably not. Most dry sump recip engines still rely on gravity for
ensuring that the lube oil eventually drains to a location near the
scavenge pump pick-up(s). If you took a recip engine with a dry sump lube
system that was designed to operate with the crank horizontal, and
operated that engine with the crank vertical, the likely result would be
that the bottom end of the crankcase would simply fill up with oil. The
crank, rods, etc. splashing through a pool of oil would cause massive
windage losses and overheating/foaming of the oil. Not to mention loss of
oil flow in the pressure pump circuit.
The only exception I can think of are some recip aircraft engines that had
lube systems designed for sustained operation inverted or with negative
G's.
This may be slightly beside the point - but the bigger Honda four-stroke outboards are based on their various car engines standing on end instead of horizontal. I don't know how they arrange their oil drainage or whether they are dry sump etc.
Like others have said, it will depend on where the scavenge points are. And other lube considerations also. As well as a few not so apparent mechanical issues too.
