Should synth oil be used before "breaking in" an engine?

[Ken C]

Everyone who has an interest in synthetic oil has heard that it should NOT be used until the engine is broken in (or else the engine, because of the oil's superior lubrication, will never break in properly). However, we have all, lately, also heard that MB (and other manufacturers such as Porsche, GM on their Corvettes, etc.) has started delivering their new cars with a factory fill of synthetic oil (e.g., Mobil 1).
I've used synthetics for over 25 years, but always after a generous break-in period.
What is the best procedure? I'm starting to have an old engine (M110), on a used car I recently bought, rebuilt (including honed cylinder walls and new pistons, cams, etc.), and planned to start in off (during break in) on regular oil, switching to synthetic oil later. Now I'm wondering if I should start the "new" engine on synthetic oil right away -- any comments?