When you say "they" I assume you're talking about a dealership. Chalk up one more bit of BS from a Harley dealership.
If the Amsoil you're talking about is the motorcycle specific 20w50 that says right on the bottle it will work in all three holes, then it is every bit as good as (and likely better than) the SYN3 that came in all three holes from the factory. So yes, if that's what you really want to do then you can run it in all three holes. However, I highly recommend using a real gear lube (75w90) in the transmission rather than "fortified" motor oil like those that claim they will work in all three holes. Your trans will thank you as you pile up the miles.
Jerry
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Got it, That is what I will do. This makes sense. I am asking because I want the best to make it last the longest.
Thanks