Some people say a synthetic does not allow the engine to break-in properly, which makes sense.
That is an old wives tale.
Break in Oils
Some people claim that you can't break in an engine with synthetic oil,or castor oil. Nonsense. I can seat the rings on a KT100 running Amsoil Saber at 16:1 in a couple of laps. Run what ever oil brand & ratio you plan to use in an engine from the very first start.
Fuel mixture is much more important. Run a little rich at first. Re-jet, use partial choke - whatever it takes to avoid running too lean on a fresh engine. This will reduce cylinder temperatures while helping to wash away the metal particles produced by ring seating.
There is a popular 2 stroke import engine out there with all sorts of break in recommendations. Extra oil, etc. The factory oil advise about 16:1 initially is ok, as long as you add fuel too. That engine is a low quality, inexpensive unit. I have measured cylinders at .005 or more out of round. Piston rings that only make contact at a few spots initially. No wonder they tell you 16:1 - the extra oil helps seal the rings! Running this engine at WOT with enough oil & fuel is the best way to seat the rings - and it will still take awhile...
About 2 Stroke Engine Break-in
I have had may responses to this page asking about breaking in a 2 stroke engine.
Any engine with piston rings takes a little time to fully seat the rings. Once the compression rings are making full contact with the bore, a 2 stroke is good to go. Most professional engine builders agree that short runs at full throttle are the best way to seat the rings. Cylinder pressure forces the rings in to contact with the bore. Higher cylinder pressures happen when the engine is working harder.
Why "break -in" at all?
Good question. Stihl, Husqvarna, Poulan, Ryobi & Homelite make millions of units each year. None of them come with highly detailed instructions to break them in gently, or use more oil. A new chainsaw comes out of the box, and goes right to work at 10,000 RPM & full load on the cutting chain. A 2 stroke race bike rolls off the trailer with a fresh top end - and goes WOT across the desert through the gears.
There are some things to avoid with a mixed fuel 2 stroke engine:
Excessive idling. Little to no cylinder pressure, and reduced fuel / oil flow per revolution. On a motorcycle, you also lose cooling air flow.
Engine braking. Pull that clutch in on long downhill runs. High crankshaft speeds with the throttle closed starves the engine for oil. Engine braking with a 2 stroke is best done with a compression release, and the throttle wide open.