I dont know if the guys over there are serious or not about the use of an additive but the G80 does not need it, but it will not hurt it at all. The additive is simply a friction modifier and many fluids come with it already in the fluid and those fluids are compatible.
It sounds like you might need to pull the drum off of that one side, probably both to compare what you see. Seems like you might have something broken in the brake assy, maybe a shoe return spring. On a dual servo brake, without the return spring working if a shoe hangs on the drum it will make it grab and more than likely let go over and over again, kind of like you are experiencing. If someone is doing the work for you then have them check it out ASAP.