Yes the break in procedure for different types of engines is different.
For ABC (Aluminum piston in a Chrome lined Brass sleeve) or ABN (Aluminum piston in a Nickel lined Brass sleeve) engines (like the 46 AX), you want to run a couple tanks through it set on the slightly rich side of lean. Run it for a couple of minutes to get to operating temperature, then shut it down and let it cool. Repeat. Generally, a couple of tanks like this is sufficient to get to the flying stage.
For a ringed engine like the 50 SX, run the engine slobbering rich for nearly a tank, then start to lean it out a little. Let it cool off completely between tanks. For each consecutive tank, run it a bit leaner, but richen it periodically to keep the engine cool. To properly break in a ringed engine can take 10 or 12 tanks at least, but you can usually do it in the air after 5 or 6, but comtinue to run it quite rich.
The reasoning? An engine with a ring requires that it be run on the cool side during break in so that the parts can "seat", or wear in together. Running rich will carry away most of the combustion heat with the excess oil and fuel that dumps out the muffler, thus keeping things cool.
An ABC (or ABN) engine is manufactured with very tight tolerances. The sleeve is actually tapered so that there is NO clearance between the piston and sleeve at top dead center (for compression). Which is also why an ABC engine is very hard to turn over by hand, and sometimes even with a starter when brand new.
Running an ABC engine rich will result in the piston (and/or sleeve) wearing too much, which will result in a fairly rapid loss in compression. Running it leaner, and hotter, will allow it to wear in much more slowly, keeping compression high. Just don't run it too lean!
OS instructs people to run the engines slightly rich so that they "break in" quicker, which is essentially what I described. I use to describe a different procedure for ABC engines, but have caught flack for going against OS's instructions, so....
Good luck with those new engines!
Dennis-