Originally Posted by Mike Hunt
what is the best kind of oil to use for break in?
Anything but synthetic. The reason being is that synthetic oil does offer superior lubrication and this can prevent the rings from achieving a proper seal. After the rings seal synthetic is fine and even desirable.
Here is some information that supports this thread. About 3/4's down the page he says No Synthetic Oil as well.
Break in Secrets
There is actually 2 stages of breaking in a 2 stroke bicycle motor in my opinion. There is the first 40km or so that are crucial. This is where the rings are seated. The other break in point comes many hundreds of kilometers later. At some point you will notice that the engine is running smooth and strong. Stronger than when it was new. This is the second break in point and you want it to happen many hundreds of kilometers later. On my last motor I think I noticed this at about the 700km mark. The longer it takes to reach this point means the longer the engine is likely to run before it will require rebuilding.