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A straight weight oil, such as SAE 80, will thin much more, than a 80W90, as temperature rises. BMW's transmission designer/manufacturer, Getrag, specifically designed the transmission to use either straight 80, straight 90, or 80W90 multigrade, all in only GL5.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, is modified ... Just in case you were 'guessing' here ...no, the objective is NOT super high viscosity index. Link #5: has never waivered in its Airhead motorcycles' transmission oil recommendations, and this includes the last Airheads. My article is VASTLY more complete, particularly about 5 speed transmission problems. The article does describe the broken clutch lever pin problems, & you will find the the exact part numbers & information in my own for your Airhead. Duane's article does not cover a LOT of what I do & certainly does not cover all my testing, nor how to go about any ideas of overhauling a transmission, shimming it, nor ANYthing about the 17.5 gear angle change, etc.
One such additive is made by Dow Corning and sold, $$$$, by bearing sales companies.Restating this: One difference is in spin up & spin down time for shifting. I am also concerned that, when colder than approximately 90F (or maybe somewhat more) air temperature; and/or the engine not being used at high output (which produces more heat, that DOES get to the transmission), lubrication is possibly reduced, protection could be decreased, & there are other not-so-nice things.For example, using 80W145: The oil at any normal operating temperature will ALWAYS be thicker than if the oil was any lighter grade, including 80W90. Shifting smoothly--preloading the shift lever and why. I am putting 5 links at the beginning of this article to help with understanding. MORE throwout bearing information is here: folks have problems understanding, let alone visualizing, how BMW transmissions operate.