The only thing is relevant is to get the load on the piston rings in first few hundred kms. Rest is nonsense. If your bearings are wearing in, then you have built it wrong, and they would not last long.
The running in process is a left over from two centuries ago when engines where chiselled out of lump of cast iron.
There are plenty of examples floating on the internet, here is a good (although painful to read) site:
http://www.mototuneusa.com/break_in_secrets.htm
Specifically this:
The piston on the right is full load vs gentle on the left.
As for synthetic, it depends what you call synthetic. Most of mineral oils are very close to synthetic these days grade for grade. 5W30 will definitely be synthetic blend. 10W30/40 will have some synthetic additives. 15W40 will probably be of a mineral kind. A lot of cars come with synthetic oil from factory. These engine last about 300000-500000kms, so it is good enough. Some engines you are not supposed to put anything thicker than 5W30 anyway, and you cannot get that grade in non-synthetic variant.
modern mineral oil (eg 10W40 variety) will not break down during the breaking in period (eg <500km), so it will behave just like 10W40 synthetic equivalent.
