Interesting point, on my airplanes with the exhaust located at about 1" of the bottom.of the fuselage (always using 23% oil) it has never stained with oil any side of the fuselage just the stabs when the fumes open like a cone, never above the stabs, and it is always straight , there is about 5" of prop distance from the exhaust to the prop tip, never seen it spiraling in any way, and smoke is the other indicator used in wind tunnels.

Very interesting..