For the Lycoming series its horsepower, not fuel flow. In fact, every
plane I've ever flown from Bonanzas to J-3's have burned 1/2 of 10% of
its horsepower (200hp burns 10gal/hr 65 hp burns 3.5 gal/hr, 250 hp
burns 12.5 gal/hr). All this same for carb and fuel injected.
The difference between an IO-360 and O-360 is 200 hp for the IO and 180
hp for the O. The 200hp burns about 10 gal/hr in cruise and the 180hp
burns about 9 gal/hr.
Otherwise the IO-360 and O-360 are 100% the same engine. Stick a FI
system on the 360 and you get an extra 20 hp.
-Robert