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I also said that you could use 75W90, 80W90, or 85W90, depending on weather. In the past I recommended you use ONLY a quality GL5 oil in your Airhead transmission; preferably in grade 80W90 for most conditions.I do not recommend ANY additives except in very special circumstances.
My present recommendation is that, if you want to, you may use a synthetic gear oil of good quality. I recommended that you do use them unless: you live in an area where the temperature that you start the bike at will be ~100F or more (that also means the un-started engine temperature generally), and that you will be riding fast &/or with heavy loading, or pulling a sidecar or trailer ...in very hot weather ... There is an exception; some transmissions will stiffen-up gear changing when the oil is hot. There are oils available that replace the "90" with 120 or even 145.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).
That does not mean that they should always be used.
I also have NO objection to Castrol 80W90 GL5 oil in -synthetic; NOR do I have any objection to any of Spectro's GL5 rated gear oils, in appropriate SAE viscosities.