10W40 is the right grade for the UK.
top ends generally fail due to poor oil pressure (level too low or oil feed too limited) or cold-start wear. the latter is wear that occurs when the engine is started from cold and is accelerated by thick oil at low temps(heavy oil with narrow temp range eg 20W30). esp in winter, warm the engine gently. I prefer to keep the bike upright to ensure the oil pick up is immersed, but this should just be superstition ;-)
10W40 is hard to find in eg Algeria so you have to be really gentle starting the bike with 20W30 after a cold night and it sounds "thin" at high temperatures during the middle of the day.
I can't comment on when Dommies go bang, but I hope not to find out to soon!
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