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 do not recommend ANY additives except in very special circumstances.For a synthetic oil, I am presently recommending only Spectro brand gear oil, in 75W90 in the version version called "Platinum", for the transmission & the rear drive of Airheads & Classic K bikes.
In some instances seals & the synthetic oils may not be perfectly compatible.
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.
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.
Link #5: has never waivered in its Airhead motorcycles' transmission oil recommendations, and this includes the last Airheads.