Personally, I wouldn't put any oil additives in the engine. Having a "wet" clutch could cause issues when adding "friction modifiers" to the oil. I am a fan of Lucas oil in my old jeep though. Lucas does make a motorcycle specific oil and I'm thinking about putting that in for the next oil change.
One of the big items that I've seen alot on these forums has to do with getting to the pilot screw and making the bike run a little richer by turning it out 2-1/2 times once you get it seated all the way. Whatever altitude you're running, you'll definitely want to make sure it doesn't run lean because that'll make it run hotter and shorten the life of the oil. Another thing with the oil is that you'll want to check it REGULARLY. When I run 50+mph for an hour it burns through about 4oz oil in about a tank. During the break in period there will probably be ALOT of metal shavings in the filter (check with others, take pics, etc., and post with your first oil change to see what others say). Follow proper break-in for the bike will seat everything good from the get-go so it last as long as possible. A Lizard breath (member here) oil cooler is on my to-do list.
The stock chain is a non-o-ring type. Lube it often (for me it's usually once every two rides). Some people don't talk trash about the stock chain but alot of 'em do. Do your research and go the direction you want. I have about 5,500mi on my stock chain now and it seems to be at about 2/3 of its life due to the previous owner not keeping up on maintenance.
Mr.Bracket (forum member) works for Procycle. They have some stuff for our bikes.
Yamaha TW200 Parts and Performance - ProCycle
Most of the people on this site are AWESOME and will help out with questions. For any not so nice response you get, remember, everyone has an opinion and like bungholes; some aren't as clean and obviously stink.
Keep in mind that the little engine guard that are on these things are basically a cosmetic add on. It bends really easy and will not protect the engine if you hit anything moving faster than you can push it (my opinion and shared by many).
The stock front tire is terrible in most peoples opinion after they've changed it to something else.
Welcome to the forum, best wishes with this fun bike and be safe!