Thanks for the information hughes. I rode the bike today for a couple hours, and everything seems to be working great with the Castrol 10w30. The bike hasn't been ridden for quite some time, but the motor was rebuilt and suspension was redone by ACME before it sat in a garage for a few years. It fired right up and ran flawlessly, so I was happy to see no leaks or any other issues. I'm not sure if it was Team Green's modifications, my lack of 500 experience, or just the fact that it's an almighty KX500, but I was pretty impressed by the power of this bike today. The bike is geared at 15/45, and it rips all the way to top speed like nothing!
As far as the NGK B9EVX spark plug, I found out this is what Team Green was running when the KX500 was still in action for professional desert racing. This bike was taken out of the racing circuit in 2005, and I think it had the same plug in it that it was retired with! I think NGK B9EVX is now replaced by BR9EIX, but I will have to verify.