Castrol 0w has different chemistry depend on version and market, it's the thing, specially A3/B4 most are not true synthetic or even mid-synthetic.the 0w-40 A3 contains about 50% Pao, the 0w-40R contains over 70% Pao
Semi Synthetic <> mid true synthetic oil, not full true synthetic base oil.What in the name are you trying to say? They are not Semi Synthetic oils. No MB229.5 or BMW LL01 can be Semi Synthetic.
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 synthetic 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-synthetic or even lower editions. Price/quality.