I just broke in an engine with castrol 10-w30  then switched to amsoil 10w-40, seemed to work great. I also ran the amsoil filter (more than twice the size of stock)

engine ran a bit cooler, however you may develope some leaks on an engine with that mileage, I read that the cleaning properties of synthetics can work a little too well if an older engines seals are gunked up from the years of conventional oil deposits.

I did switch from valvoline in my honda accord (175,000 miles, zero leaks) to mobile 1, and only developed a seeping, not quite leaking effect from the valve cover onto the exhaust manifold, but only when the car sat for more than a week or so.

I say if your engine has zero leaks, or easily fixable ones, go for it. I have never heard a bad thing about Amsoil from anyone, and the poeple who use it usually swear by it.