A2 no longer exists, its only A3 now. It is accompanied by B4 also, thus A3/B4 is the current spec.
It doesnt 'imply not using synth unless it meets spec' it means you can use either fully or semi, so long as they meet ACEA A2 or 3.
Castrols recommendation of 5w40 is correct as it covers a wider operating range, as I explained above.
Oils have moved on, back in,the late 90's 5w40 was still pretty fancy, as was any oil covering a range that wide, hence 10w40 or 10w30 were common. Nowadays 5w40 is almost the new 20w50, inexpensive and suits a whole lot of engines from the previous generation and the generation before that too. Fully synth, despite the old wives tales some people still believe, is simply better than semi, and no piston engine 'needs' semi or 'gets better protection' from it.
So as said, if book says 10w40, you want 10w40, 5w40, 0w40 A3/B4.
If book says 5w30 you want 5w30 or 0w30, usually A5/B5. (Replaced A1/B1)
10w30 has no place in the modern world really, hence it is pretty scarce.
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