I had 5 litres of (a quality brand) new 10w/40 semi synth in the garage that I didn't want to go to waste (Correct spec JASO etc) so put that in. Seems absolutely fine, maybe ever so slightly quieter running (less mechanical noise). I have read through this and many other oil threads TBH and they are all the same. Many understandably stick to recommended weight only, many don't over think it, many do (me for one). It only seems to really matter if you happen to live in a very hot or very cold country where temperature extreme's are found.