You can use them anytime. As far as the best, that will be at the opinion of everyone. I personally don't run additives, and luckily I've never had issues not running them.
Most people are getting that lag.. as far as the settlement information, it just takes time. I'd recommend keep calling and ask to speak to a supervisor to escalate your claim.. exhaust all resources.
https://jalopnik.com/2020-ram-1500-ecodiesel-fails-to-achieve-best-in-class-1838843741?fbclid=IwAR3BuxZqKHjO3Rtx9uoo4em_wYzoXQfPzVpPrFSjarrNwzQ2zhq-4vyM84s
It is pretty funny how Ram talked up a big game, but then did end up "losing". But, 1-2 MPG is not a ton.. Long live these half-ton diesel...