Here in Europe there's a Castrol Classic XL 20W50 engine oil available, I cannot find this for the US market, don't know if it's available in the US.
https://www.walshlubricants.ie/product/castrol-classic-xl-20w50/
It's intended for car's from the 60ies up to the 80ies.
The Castrol GTX (3) blend was
afaik developed to fight against sludge in the engines which was a problem in the 70ies to 90ies. I expect the new GTX 20W50 to be designed for cars of same period as well, only with higher viscosity for cars like the aircooled 911. The highest GTX viscosity I know before was the GTX 15W40. There's another GTX 20W50 for high mileage engines according the Castrol worldwide website. Probably also a consequence of higher temperatures due to the climate change...!?
https://www.castrol.com/en_us/united-states/home/motor-oil-and-fluids/engine-oils/motor-engine-oil-brands/castrol-gtx-brand.html
They slightly differ in their values according to the datasheets. Unfortunately no more detail about the additives and the purpose.
When you read on the Castrol website it's clarifying a bit more:
WHAT ARE THE BENEFITS OF CASTROL GTX?
Superior protection against engine sludge*
Advanced protection against viscosity and thermal breakdown
Premium quality base oils and anti-wear additives to help extend the life of your engine
Helps minimize oil consumption
*When tested in engines with a high level of sludge
WHAT ARE THE BENEFITS OF CASTROL GTX HIGH MILEAGE?
Phosphorus Replacement Technology to help extend the life of the emission system
Superior dispersancy that fights sludge build up
Exceptional oil burn-off protection exceeding industry standards*
Seal conditioners to help reduce leaks
Advanced additives that minimize engine wear
Improved fuel economy**
GTX ultraclean is for modern engines and is a synthetic oil. GTX fully synthetic and Edge are synthetic oils for modern engines as well.