The thing is the synthetic will get just as dirty as conventional ATF. I think it's better err on the low side rather than overfill so stick to the recommended amount. I leave mine about an inch below the level progressives call for.
Agreed Dave, I merely believe synthetics have greater shelf life. Not sure, just opinion.
Dirt, or carbon from heat, collects mostly from usage and somewhat during storage, but much less so. When I only put 1500 miles on my GL1200 per year, and other miles on my GPz900 or my R-bike or my K-bike, its not like I'm servicing ONE vehicle that get 20K miles of punishment per year.
I believe one advantage of synthetics, is you can go longer between changes. For years, I thought synthetics were nonsense. I drive mostly old cars and bikes, that were designed to use petroleum based products. Why pay 4x for oil, if you are going to change it 2-3 times per year. Why? Don't - it makes absolutely no sense.
Like so many for-sale ads; "Changed oil every 2-3K miles with Mobil-1." If you use a superior oil, why do you change it so often? When I drive a car 20K miles per year, I change oil often and use what the manufacturer calls for.
I will likely leave this AMSOIL Synthetic ATF in my GL for 6 years. I'll bet it looks pretty good when I drain it next time, because of how anal I cleaned the forks!
(Job 99% done. I’ll take it for a ride tomorrow and then park it the other direction for the new rear tire!)