Read this thread on additives:
https://forums.noria.com/eve/forums/a/tpc/f/616604995/m/8511066601
One question is whether you need an additive or why you think you need an additive.
Also, for the rear end, tranny, and power steering, I like to use Redline Oil or Specialty Formulations' oil whenever applicable as it is heavily additized.
I do play around with additives for engine oil. I like Valvoline Synpower Oil Treatment for its low cost and good components (see details in thread above).