I should have mentioned my car engine experiments, but I was in a forum of airplane guys so I did not.
With the tight little, high reving car and buggy engines, my opinion is that running 4-stroking rich is detremental beyond a shadow of a doubt. Not too often will one spit a rod, but the difference in how long the "design fit" or pinch lasts is extremely dramatic.
Even 4-stroking rich on some of the very high performance car engines will get to the high 20K's in rpm's, with full peak performance in the high 30K's to low 40K's. I think that the high reving nature of these accelerates the pinch wearing away quicker than an airplane engine, the rod also likes to egg shape the crank pin journal when the engine is run for too long 4-stroking. Now lets aggrevate the situation by using low oil car fuel (typically 10% to 14% oil). It is quite comnmon for a car engine to only last 2 to 4 gallons of fuel when broken in to the "common, accepted" method of ideling slobbering rich for several tanks where it is so cold you can hang on to the head with your hand for the whole run.
When this is done, you notice the pinch is dramatically less than when you started.
With the car engines, I have demonstrated this to a "car engine tuning / break-in class" I hold a couple times a year at near by shops, by breaking in two OPS 12's (not cheap engines) at the same time. One I fire up for the first time and run the snot out of it in a rich but very clean 2-stroke, the other one sits and idles slobbering rich while I'm running around the other car. Its funny, the crowd thinks I'm going to completly destroy the one I'm running hard, inside a couple tanks and the one sitting ideling like the instructions state; they think will be the best.
After about three tanks ideling and three tanks on the other one running around and slightly leaning each run, we did a pinch check. Guess what?- the one that sat ideling the pinch was nearly gone, the one run around it was nice and tight. This is why car engines are only warrantied for 30 days, not 2 or 3 years or even 5 years like one plane engine manufacturer.
Guys that break in a car engine the ABC way, usually are in disbelief untill they realize that their engine is lasting 10 gallons of fuel instead of 2,3 or 4 gallons between rebuilds.

I once talked to the exclusive US importer of a very high quality, high performance (exceptionally expensive) car engine line from Europe, asking the owner why he recommended such an excessivly rich break-in, telling him that the stated method will prematurely wear the engines out. He told me laughing "Thats why these engines carry no warranty, for every engine I sell, I will sell two piston / sleeve/ rod sets" and further went on to say "the car crowd has been conditioned to wear these things out so the parts business is real good,....If we told them how to break-in and run these things right, I'd be losing a lot of money"

lets keep the flames to a minimum folks.