The"Professional" line is just what a dealer uses and might be more tailored to a specific make.

Some of the full synthetics that Castrol sells that are US based, actually starts as crude oil. Then you have them pricing it as a real synthetic even though it is not. The use of crude oil as a base stock is cheaper and Castrol on record even admitted they wouldn't drop the price of a product that was now switched to a crude base stock being marketed as a synthetic.

I'd prefer to use a real synthetic oil from a company that doesn't play games about what their product actually is. I'd prefer Shell way over Castrol. Rather than spending money on a team of attorneys trying to change the meaning of synthetic, I'd rather spend my money with a company that put that money towards science to produce a better product; like what Shell has done with the GTL technology.