additive if your transmission is filled with synthetic oil.One such additive is made by Dow Corning and sold, $$$$, by bearing sales companies. There are oils available that replace the "90" with 120 or even 145. I also said that you could use 75W90, 80W90, or 85W90, depending on weather.Any oil will thin & thicken with temperature changes on 'its own chart curve'.How this happens is a property of the base oils & additives.I have no objection to it being used in the driveshaft (of those models using oil there).
For such as a 80W145, as just one example of oils with a much higher top number; as your parts & oil rise in temperature, the oil thins less, & the specific lubrication qualities of friction inherent in thicker oils, are modified; ...and, note what I mentioned about speed of parts slowing-down or speeding-up being changed from AS DESIGNED. Spacing of parts, spin-down and spin-up times, etc., all change as the transmission heats up; and the oil grade is specified to help match other transmission characteristics.Any oil needs to be of a type that leaves a very thin layer that sticks to the gears & bearings rather than drip completely away during overnight or longer storage.This is particularly so for the 5 speed transmission.If you wish to think about this in a different way, imagine the oil is rated at 80W20000, & THINK about what that means.Thus, at most any temperature you will be riding at, even after a full warm-up, the oil is thicker, a lot thicker. BMW has never waivered in its Airhead motorcycles' transmission oil recommendations, and this includes the last of the Airheads. My article is more complete, particularly about 5 speed transmission problems; but, Duane's article has things I do NOT cover. NOTE that Duane's article covers mostly the /5 and /6 era transmissions, but a lot of the information is applicable to ALL Airhead transmissions, including the /2 era. While there can be other causes that a change to a thicker oil will not help, in this instance you can try the thicker oil, such as 85W140 or similar. 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 few transmissions will stiffen-up gear changing when the oil is hot.If you change from a petroleum to a synthetic, & get some weeping or leaks, change to another type of synthetic, or go back to a petroleum.With the change back to dino oil, the seal(s) are then likely to reshape with some miles & time and stop weeping.