re: OT mechanics how long does synthetic oil last

Oil is oil and doesn't "break down" what it does do is get dirty, lose its detergent abilities and pick up oxidants and acids. The benefit of synthetics is consistency of the molecules that make up the oil. Think of it like a bowl filled with marbles all alike in size and texture as a synthetic. Then another bowl filled with marbles of different sizes and consistencies as the non synthetic. The ability to do the job is more consistent and long lasting in a synthetic.
Read Bob The Oil Guy and you will find an oil that gives you the additives needed for your engine and type fuel. It might surprise you to find that the WalMart brand synthetics hold up well in the quality discussion and that some of the "premium" brands like Amsoil and Royal Purple have some qualities that are superior are often times just a lot of hype given their price.
I change at the half way point personally. To me the cost of an extra change a year is cheap insurance when compared to the cost of an engine failure. Some disagree.