Synthetic Break in (I know its an old thread but I'm new)
I just wanted to weigh in. I've been a professional small engine mechanic a looong time and have broken-in and seen a lot of small engines broken in, so I may have more experience than average. I've tried dozens of recommended break in procedures from almost as many manufacturers over the years. I don't think there are many hard/fast rules. Internal engine tolerances and materials can vary widely and a Chrome-cylindered engine breaks in differently than Iron or a Aluminum does. Ring composition, bearings, even the shape of the piston skirt comes into play. Ambient temperature makes a difference as well. Here are my thoughts:
1) The less material you remove braking in an engine, the longer it will last.
2) Most engines break in best/quickest when the operator constantly varies load and engine speed.
3) Most engines should not spend much time in high vacuum states during break in. (Engine Breaking)
4) The more poorly manufactured the engine, all else being equal, the longer the break in.
5) The break in oil should be the same type of oil the engine is run on after break in. Non-synthetic oil manufacturers spent years trying to misinform the masses that synthetics "cause" everything from unseated rings to leaky seals. Now that they all manufacturer their own synthetics, they are suddenly less strident. IMHO the main advantages of synthetics as seen in the field are consistent TBN (viscosity control) and high temperature stability, which allow them to protect dramatically hotter engine parts well past the point that regular oils would have turned to smoke and carbon, not super-duper engine-ruining slipperiness. Just my 2 cents.