1. 0w30 have had problems with main bearings (specifically, the 99-03 m3 motor had manufacturing faults) BUT:
DEALERS (READ WARRANTY) USE 5W30 FOR ALL 6 CYLINDERS IN THE U.S. NOT REPEAT NOT 0W30.
one has to be very careful, becayse the film strength in synthetics is very different, despite similar viscosity ratings.
2. castrol synthetics i.e. 5w30 or 5w40 have almost the same film strentgh, and vary minimally by the hot (30 versus 40) viscosity modifier, which is a different polymer in these two oils. the synthetic base is liklely the same, although i have not seem their formula (or gave enough shit to mass spec it myself).
hence yes, it is ok to use this particular oil whether 5w30 or 5w40.
i would be VERY hesitant to be guided by recommendations of a british parts store (halfords??) in this case.
3. mobil 0w40 is i think what european dealers use, although i would be very interested in the "oil cartel dude" to comment on this.
mobil few years ago reformulated its synthetics, which i find to be better than castrol. mobil has NO ASH (which is definitely present in castrol synthetics), and has very good hot film strength (the are confident enough not to add zinc to their formula).
i do not know the difference between their 0w versus 5w, but unless you are in siberia or minnesota, i would go with 5w.