It is truly amazing to read about the gimmicks and "special sauces" some dealerships try to sell to customers. It is also amazing to talk to many people, only to find out... They have never opened and read through their Owner's Manuals.
Decades ago, I think (Yes, this is just overt opinion) it was easier to read through one's Owner's Mannual, as topics and references were better aligned within it. Today, you read one section starting on page 251 (which is very generic info.), but the information you are honestly desiring is not located under that section, but rather on pages 379 & 451 (specified and/or specific), yet the chapter is not what one though it was under. Often we see that the latter half of our manuals have the service intervals and the specifications we are looking for followed by all this extra electronics information. Years ago, finding answers to one's maintenance queries, within the Owner's Manual, was easy... You found all you wanted in the back/latter sections of our Owner's Manuals. This jumbling of Owner's Manual information only serves the companies and force us (the consumer) to say... "Ah, the hell with trying to find this or that in our books/manuals". Thus you show up at a dealership for services. (Incentive & Money)
For years, I have read over and over... FOMOCO does not advocate using additional chemical treatments or additives for this or that. The major exception to that are the limited slip differentials found in certain models, and even FOMOCO has that covered. They sell their own specific additives.
I might have been rambling but... Service intervals & maintenance cycles should be based off of driving conditions. Some dealer sold additive, not recommended by the OEM, should indeed be questioned. Ask them if they would cover any future repair regarding the use of said item/additive, and if the Service Manager, General Manager and the dealership owner would sign a document attesting to such a claim (when sold items that are contrary to OEM recommendations) so that you may have that documented for your personal records... HA HA HA HA! That shuts them up quickly too, so I have experienced & learned. (Yep, have never forgotten those experiences.)
AT LEAST TODAY, I HAVE A DIFFERENT DEALERSHIP THAT I TRUST, HAVE DEVELOPED AN EXCELLENT RAPOR WITH & HAVE NEVER HAD AN ISSUE WITH THEM AT ALL!