The builders I talked to all recommended to stay away from hardened seats, unless you use the motor really hard.
I also heard about poorly installed seats that came loose eventually!
I used CD-2 lead substitue for the first few years on my daily 235, but haven't added anything to the gas in many years now with a fresh head.
I would stay away from any additives in fuel and oils with an engine that runs well without any issues.
I'm also not worried about hardened seats on my current 261 rebuilt.
