OVERKILL
$100 Site Donor 2021
The point is, you can't just go by the grade. You are claiming 10W-40 is the better option, but the reality is that many of the 10W-40's are cheaply blended oils with no higher concentration of AW additives than their 5W-30 siblings, despite being permitted to have them under the API regs.If choosing between Mobil 1 5w30 and Mobile 1 10w40 HM, or Amsoil Signature Series 5w30 and 10w40? (actual oils 80s/90s Corvette owners choose between)
10w40 every time
The Euro 5W-30 I shared above is a prime example of a 5W-30 that's better than the 10W-40's listed, because it has to pass some demanding, and current, OE approvals (BMW's in this case), while the 10W-40's don't, because nobody calls for that grade. This is the approval table from Mobil, note there isn't a single X in the 10W-40 column beyond the standard API one:
Interestingly, the 5W-30 has a superior flashpoint to the 10W-40 too:
The 10W-40 also has API constrained levels of phosphorous, despite not requiring it, which, as you'll note, is considerably lower than the BMW 5W-30:
If you are hell-bent on using an xW-40, I'd be far more inclined to use a full-SAPS Euro 5W-40 or 0W-40, which will be a more capable lubricant.