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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.
My present recommendation is that, if you want to, you may use a synthetic gear oil of good quality.
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.
Any oil will thin & thicken with temperature changes on 'its own chart curve'.
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.
My article is VASTLY more complete, particularly about 5 speed transmission problems.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.If you change from a dino to a synthetic, & get some weeping or leaks, change to another type of synthetic, or go back to a dino.
With the change back to dino oil, the seal(s) are then likely to reshape with some miles & time and stop weeping.
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.