that topic is alot about information + marketing kf the product. I read that topic till the point where he wrote the 5w means the oil is thin. I think this gentlman seriously lacking to inderstand the viscosity of the oil and its relation with temperature.
Look. If the manufacturer recommends to use 10/40. Then we can use
10/40
15/40
15/50
20/50
these oils will work fine.
But if we will use less thinner oils like
0/30
0/40
5/30
5/20
etc. The engine will still work. But it will not perform well because we are going against manufacturer's recommendations.
However, if use 5/40 oil, then it will be pretty acceptable. But important is that 5/40 oil is fully synthetic and comes with api sn grade which is again not recommended for bikes.
So make sure that 5/40 should be old grade like sg, sj, sl, then its fine.