Regarding retailers, I didn't even know what TSC stood for, so I looked it up. Looks like a staple for much of the country, but there is not a single store in the Pacific Northwest states.
Sorry, we just assumed.
Perhaps you can find a retailer for Fleet Charge in your area. Conventional Antifreeze in Heavy It's convenient for me to get, and fair priced. (nearly the same price as Prestone). So, I use it.
It would have been nice if they would have given us a good reason for changing it annually. It leaves things open to speculation as for why. We know antifreeze will last a lot longer than a year. That leaves the need for more diesel coolant additives, (DCA), or as some call it supplemental cooling additives, (SCA), as one possibility. Diesel antifreeze will be formulated with the proper amount of these in it. And, could have the potential to last longer than automotive antifreeze in this application.