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There is no question in my mind, that HIGH QUALITY SYNTHETIC GL5-rated oils will, or can, PROLONG the life of your transmission (& rear drive).
I recommend that you NOT use any additive if your transmission is filled with synthetic oil.
One such additive is made by Dow Corning and sold, $$$$, by bearing sales companies.
I have no objection to it being used in the driveshaft (of those models using oil there).
In fairness, I will also note here that one experienced transmission rebuilder, Tom Cutter, my words here, believes synthetic gear oil in your Airhead is not good, leading to unspecified WEAR.
Link #5: has never waivered in its Airhead motorcycles' transmission oil recommendations, and this includes the last Airheads.