It won't hurt anything to convert back to castrol. That's an old myth that's been disproven. Whether synthetic is better, I don't know. On paper, it is better, but in practice, there are plenty of opinions, but not much in the way of actual proof. If the manufacturer recommends synthetic, why wouldn't you go with that?
My own Altima has 241,000 miles and still runs great, good compression, good power, no smoke. I use Castrol. However, it's not a Turbo.
