I've been using Motul 300V in my Royal Enfield for a few years now and can vouch for the positive impact it has had in terms of slicker gear shifts and lesser engine vibes. This is not a racing bike by any stretch of imagination.
Now the facts - As you might know, 300V is a fully synthetic oil, as opposed to semi synthetic and mineral oils. What this means is that this is a oil 100% born in a lab as opposed to a SS or MO. A lot of research has gone into 300V type of products to perform the way they do at their specified operating ranges.
Coming to the operating range, the 15W specifies that it maintains it's viscosity to specified standards at winter temperatures of 15 deg C. the Higher number specifies the viscosity standards at temperatures upto 100 deg C. These two operating numbers is what makes the oil a multi-grade oil.
So a 5W30 oil will have a better performance at even lower temperatures than the 15W40 but will thin out more than the 15W40 at temperatures approaching 100 degrees C. That's all there is to it.
The specified standards here could be OEM - BMW, Merc etc or industry body based - SAE (Europe), API (USA) or JASO (Japan).
The rest is all marketing fluff, and in some cases like Shell's Helix offering there are genuine additives like detergents to keep the engine internals nice and sparkling.
The other USP of 300V is just the opposite of what you've been told - it supposedly lasts a very long time compared to other synthetic or semi-synthetic oils (10000 miles or so). I personally don't trust this one and religiously change my oil every 5000km.
Hope this helps and as always - YMMV.

Now the facts - As you might know, 300V is a fully synthetic oil, as opposed to semi synthetic and mineral oils. What this means is that this is a oil 100% born in a lab as opposed to a SS or MO. A lot of research has gone into 300V type of products to perform the way they do at their specified operating ranges.
Coming to the operating range, the 15W specifies that it maintains it's viscosity to specified standards at winter temperatures of 15 deg C. the Higher number specifies the viscosity standards at temperatures upto 100 deg C. These two operating numbers is what makes the oil a multi-grade oil.
So a 5W30 oil will have a better performance at even lower temperatures than the 15W40 but will thin out more than the 15W40 at temperatures approaching 100 degrees C. That's all there is to it.
The specified standards here could be OEM - BMW, Merc etc or industry body based - SAE (Europe), API (USA) or JASO (Japan).
The rest is all marketing fluff, and in some cases like Shell's Helix offering there are genuine additives like detergents to keep the engine internals nice and sparkling.
The other USP of 300V is just the opposite of what you've been told - it supposedly lasts a very long time compared to other synthetic or semi-synthetic oils (10000 miles or so). I personally don't trust this one and religiously change my oil every 5000km.
Hope this helps and as always - YMMV.