10w40 is fine. Martin's sounded loose on straight 20w when it got hot. semi/fully synth is pretty much personal choice. I'd use semi-synth.
the real issues are oil quality and contamination and that depends on your wrist action and dust/water where you're riding. you're meant to change it every 1000km, but that's for hard use. most people can safely extend that to 2000/3000km without much risk. Martin changed it after 3000km in Algeria and the old oil was still OK, but the level was rather low.
I used to run Shell semi-synth in an XR400 for 5000km at a time where 1500km change was recommended.