Castrol 0w has different chemistry depend on version and market, it's the thing, specially A3/B4 most are not true synth or even mid-synth.the 0w-40 A3 contains about 50% Pao, the 0w-40R contains over 70% Pao
Semi-synth <> mid true synth oil, not full true synth base oil.What in the name are you trying to say? They are not Semi-Synth oils. No MB229.5 or BMW LL01 can be semi-synth.
0W40 Castrol is at least 25% PAO, and the rest is a combination of AN's and Group III.
X-Max is at most 10% PAO.
MB229.5 and LL01 are even lower than semi-mid synth oil, there you have 5w30 and 5w40 MB229.5 and LL01 even with GII+ and GIII base oils without true synthetic.
Some Castrol 0w are 25% PAO yes and more, repeat, other not depend on market. This example seems not to me.
X-Max like I said have better price/quality where it's cheaper that any Castrol 0w full saps mid-synth or even lower editions. Price/quality.