I looked into this, and it seems like a decent oil.... however.... VR1 seems like a better oil, and is cheaper.
https://bobistheoilguy.com/forums/threads/hydro-oil-m1-15w-50-vr1-20w-50-or-gtx-classic-20w-50.365777/
https://bobistheoilguy.com/forums/threads/valvoline-vr1-racing-vs-castrol-gtx-classic-20-50.358576/
https://bobistheoilguy.com/forums/threads/new-castrol-oil-for-the-classics-castrol-gtx-classic-20w-50.339113/
"Overall VR1 is probably a better oil, made with a modern SP additive pack with boosted ZDDP and more ZDDP than the Castrol, also the Valvoline is mostly Group II while the Castrol is mostly Group I, I haven't seen a full VOA of the Castrol but from the datasheet I know it has less ZDDP than VR1, and VR1 has a good VOA video here, I don't know if the Castrol has any friction modifiers but the VR1 does, I can tell that at least the Castrol probably has SN levels of calcium and not as much as some older formulation A3/B4 oils and CI-4+ diesel oils since it's .95% ash but I don't know if it primarily uses calcium detergents or if it uses a calcium/magnesium blend like modern SN+ or later lubes."
I bought another 6 pack of VR1 on amazon for $42
https://amzn.to/449Zh3r
So I'm sticking with VR1, my advice is worth what you paid for it.