The NX650 came from the factory with ATF (automatic transmission fluid) in the forks. At the time of production ATF was pretty standard across all manufacturers and was the equivalent of about 10wt oil. Todays more-complex transmissions mean there are many different types and weights of ATF, most of which are of a lower weight to provide better fuel mileage. Dexron V, for example, is closer to 8wt.
When replacing the NX's fork fluid, I would suggest switching to 10wt fork oil, but this creates it's own set of problems. The weight rating of an oil is kind of a guess. As you can see by the chart below, not all oils marked as the same weight are actually the same weight. When I still had the stock forks on my bike, I had changed the fluid to Dexron V ATF, which was plusher over heavy washboard roads, but caused the forks to bottom on whoops. I then changed it to Bel-Ray 10wt fork oil, which made it feel stock again.
