put in a fresh motor (rebuilt with new parts), how long should i let it crank before it starts?

 

Turn it over for about 30secs or so just to let the oil circulate around it is up to you if you really want to do it longer. Last motor I did I cranked for 30 seconds stopped a few then repeated like 2 more times never had a problem and that was on a brand new SER spec V motor soooo....

You know your not suppose to run syn till after like the 2nd or 3rd oil change? Not saying you did just trying to prevent you from throwing a rod bearing