Perhaps you noticed that your engine has six jugs instead of four. This means that it costs 50 percent more for everything including unfortuanately, gas
15gph with an O540 corresponds to 10gph for a O360, which is what just happens unless you take special care about power settings and leaning.
You can approach the fuel burn of an O360 by lowering power to what an O360 would deliver. The extra horses of the 235 are for takeoff performance, climb, and payload... not speed. Try a 60 percent power setting and see what you get.
I have heard that full power for takeoff burns at a rate of 25 gph. I would not know how to measure this, but I sure pull power back quickly.