Oil - 10W40 is specified. Use that, or use what I use - 5W40 Shell Rotella T6 synthetic ($20/gallon at Walmart). I've never used oil additives, and I have 28,000 no-trouble miles on my Bandit using Rotella.
Gas - use a Top Tier gas like Shell or Chevron, 87 octane. Install an in-line fuel filter.
Gas additive - Chevron Techron or Sea Foam once in a while (when you change the oil). My air-cooled engines and my Cherokee seem to like Lucas Fuel Treatment on a constant basis (improves mpg by 5-7%).
Miles-to-empty - I just installed a Sigma 1106 wired bicycle computer. Cost ~$25 and took about one hour to rig. Keeps track of trip and total distance, avg and max speed, pace, clock, etc. Very nice.
Tires - I hear Metzelers are good as are Pirellis. The IRCs that came on my bike will need replacement soon. I don't think much of them. Keep the inflation correct, 29 f/33 r for solo riding.
Fork brace - this could be the best performance improvement. I'm getting one from Tkat.
Brakes - EBC FA-106 brake pads. Came on my bike. Reported to be better than OEM.
Shop manual - buy a Clymer's. Unless you know the PO did this within the last 3,000 miles you should do a automatic decompressor/valve adjust/cylinder head bolt re-torque soon. Also within next 3,000 miles you should inspect the cam chain adjuster.
Safety checks - check every nut/bolt/screw that you can easily access, especially check torque of 1. rear axle bolt (there's no cotter pin for back-up) and 2. brake/brake lever mounting points and 3. front drive pulley bolt.
Happy riding!