Hi,
it started here in the mid 1990s as GTX Magnetic - a synthetically enhanced 15W-50 lubricant. Typically as Castrol was in the throes of selling to BP it was relaunched around 2001 as GTX Magnatec 10W-40 - still "synthetically enhanced"!
Castrol lost a LOT of favour through this "sly" move in viscosity that did affect many "normal" users - the market it was intended for! It was stated to have "UMA" technology - Unique Modular Attraction as cornerstone. Wait, their is more - sorry I won't go on!
Actually the product is a good one - semi-synthetic - and is also available in a "diesel engine" version here in Australia
One of my Sons uses it sucessfully in a highly utilised [censored] turbo-charged 2.5 litre diesel engine