Essentially the 10 (instead of 15) means the oil will be thinner when cold so the bike will start better in cold whether and the 40 (instead of 50) means the oil will also be thinner when hot so I'm thinking that with thinner or at least least viscous oil in the engine when it is hot the gearbox may be more difficult rather than easier. My owners manual does not recommend anything other than two types of 15/50 so I personally would be reluctant to go to a lighter weight oil.
Weird, my owners manual recommends anything from 10w 30 to 20w50 depending on the ambient temperatures it´ll be used in. As I hardly ride in towns the engine doesn´t often get very hot so I´m confident 10w40 is OK for my use.
I´ve found careful lubrication of the gear link mechanism helps with changing, as does chain adjustment but its the false neutrals that bug me. this is my sixth Duc and none have had such a poor box.
I never have a problem finding neutral on my Scrambler, however, it's not uncommon for these bikes to have a poorly adjusted clutch levers that don't fully engage the clutch when depressed. Perhaps this is a root cause?
Same here with neutrals on our Scrambler. I was a little difficult to find neutral until it warmed up when new but once it had 600km or so on the speedo its been no problems at all either with the proper neutral or false neutrals.
If the manual recommends 10w40 where you live then give it a go oil is not particularly expensive in the overall scheme of things.