When the GL1800 was first released in 2001, the service manual called for 10w-40 oil. At some point in subsequent years (I think it was 2006) Honda changed the recommendation in the owners and service manuals to 10w-30 oil. Though I'm not aware of any changes to the lubrication system or engine that caused that change. I'm not sure what factors were involved in Honda changing the recommendation either, but I have a couple good guesses.
In some respects, 10w-30 is a superior viscosity simply because it doesn't contain as much VI (Viscosity Index Improver polymer) additive in it, which means it has a higher oil content to begin with. Some 10w-40 oils can have as much as 30% VI additive by volume, which means you have less lubricating oil to begin with, and VI additive by itself has no lubricating properties. Auto manufactures stopped using 10w-40 oil many years ago because it simply breaks down too fast, and my best guess is that this is also why Honda has shifted away from it.
10w-40 should work fine in all year bikes, but you need to be aware that it breaks down faster due to the higher VI content in it, so you'll need to change it a bit sooner. If you can find the brand you like in a 10w-30 blend, that would probably be a better choice. The only problem I've found is that it's hard to find any motorcycle oils in a 10w-30 viscosity.
Synthetic oil doesn't need as much VI additive in it to begin with, so a 10w-40 in a synthetic is also a good choice, and will probably hold up as good or better as a conventional oil in a 10w-30.