I'm not going to argue this point with anyone but I will say that
synthetic oil is simply "Too slick" to allow the seating of the rings on
a newly rebuilt engine.

I put my freshly rebuilt Boston Bob 2.1 engine in my '85 Westy in June.
  My first fill and the first several oil changes were 10W 30.  Then, at
1200 miles, I switched to 10W 40.  I now have over 9,000 miles on it and
my next oil change will be to Mobile 1 15W 50.  I already have the
Mobile 1 onhand.

With my old 1.9 engine (it was still running good but used oil at
210,000 miles) I used Castrol 20W 50 in the summer and Castrol Syntec 5W
50 in the winter.  I could've used the 5W 50 year round but it was
harder for me to get.  The 5W 50 allowed me to start it easily in
northern Maine sub zero temperatures yet it was heavy, when the engine
warmed up, so it was good for long trips at highway speeds.

Mike
Glade Spring, VA/Houlton, ME