Spot-on with the explination of the weight ratings, kx137.
The 5w40 would be fine to use in the bike.
And as explained, it will not be any thinner oil than a 15w40 will be. Even if it were, it isn't a big issue. In fact, the conventional wisdom of lubrication engineers of late is to use the thinner oils. With advnaces in oil technology, it's found that a thinner oil shows to allow an engine to produce more power, while still being able to effectively protect the surfaces. Although it is still understood that a more viscus, or thicker, oil will provide even better protection, but with a slight compromise of power.
Diesel oils have more robust additive packages than do auto oils, and most motorcycle specific oils too, for that matter.
So many rumors and tales. The molybdenum scare is another of those unwarranted rumors passed around. Folks, many motor oils do use molybdenum in their additve packages (MoTDC), as it is one of the absolute best multi-purpose additives available. But, the moly that is used in motor oils is not the same thing as the moly that would be used in chain lubes or rebuild fluids (MoS2).
The type of moly used in motor oils does not show to cause clutch slippage.
Matter of fact, most oils that do indeed have "friction modifiers" will not cause your clutch to slip. Most clutches that do slip probably already have an issue that is unrelated to oil choice.
And when we think about using a full synthetic or a mix, realize that if we are keeping the oil in our bikes long enough for it to matter if the oil is synth or not, we have left the oil in the bike way too long.
Granted, synthetic is a better choice, due to the fact that it is far better able to keep the oil from going acidic due to heat and degredation. But for the short period of time we should keep the oil in our bikes, it really doesn't matter what type of oil you use. Frequency of change is far more important. Most all off the shelf oils will perform for such a short period. Yes, some have better additive packages and are cleaner fluids, but none warrant the elevated pricing that moto specific oils carry. The moto oil thing is a load of marketing CRAP!
Yes, I use full synthetic fluids when I can, but it is more of a feel good thing in motorcycles.
Only if you are looking for extended oil changes should you be completely on board with full synthetics, and steer away from conventional organic oils.