My TT Pro 40 recommends caster during the break-in period as well, but it does say castor/synthetic blend in the previous sentence. I personally am switching to synthetic because the guys that run synthetic have cleaner engines. More important than whether you are running castor or synthetic is to make sure the engine runs rich during the breakin so it does not get too hot. Plus, the extra lubricant from running rich aids in doing the final machining of the piston and sleeve. For that matter, you never want it to run hot. I use a tachometer and set the main needle rich by approximately 300-400 rpm.

It only takes a few tanks of fuel to break in the TT Pro, so you might just use some castor fuel from one of your buddies at the field, if you are concerned using the synthetic. Then switch to the synthetic afterwards.<br type="_moz" />