I have the same engine in a different bike. The valves don't really go out of adjustment, but checking them at the proper intervals is a fairly easy job. I like synthetic 5W40 oil in the winter, but a good conventional oil works just fine. Depending where you live, the recommended 15W40 might be a little thick when cold starting. 10W40 might be a better fit.
Gear oil is easy to find at auto parts stores. I use 80W90. The specified SAE90 might be hard to find, and possibly not worth the effort.