What's the word on Shell Rotella T6 5W40. I've been using this in my 2018 F-350 6.7L ever since the truck was new but now it's impossible to find. I've heard that Shell is lacking a certain additive and thus has not been able to make the product, but I don't know if this is true or not. Anybody out there have any information on T6? I may need to switch to another heavy-duty diesel oil 5W40 like Valvoline or Schaffer if I can't get Rotella T6.