Since I've been using and selling AMSOIL for over 25 years, you are welcome to consider this as biased:

I would not, and don't, run any engine on other than synthetic. I do extend the drain interval. I wouldn't use other than Group IV (or higher) base stocks, which eliminates Castrol Syntek.

I strongly recommend using a good engine flush, such as made by AMSOIL or BG, before changing. If you don't, or won't, then go ahead and put in the synthetic. Watch the oil level--if usage suddenly increases, change the oil filter and top off. If you don't flush, you might want to drop the first load after 3-4,000 miles, as the synthetic will clean out whatever gunk and/or sludge has been left behind in the engine by non-synthetics.