As long as meets the standards that they specify they cannot require a certain brand. But they could say that 90W-250 or some other viscosity does not meet their requirements.
The dealer i use in ft.myers florida sells red line products right next to h-d products,,, if you get your bike serviced there, they will put either or in your bike , your choice
In fact they recommended redline when I went in and got oil. Only problem I can't stand the smell I get from the redline in the tranny
After 2 transmission failures my HD mechanic told me to get away from Syn3 and use Belrey gear oil and since then no problem it's quieter smoother and no more shavings on the drain plug.I don't know how Harley not using an actual gear oil for tranny.I now use Red line gear oil for last 20K + miles and very happy with the results.
I have had three shift shaft seals replace under warranty.Redline has been in the tranny all those times.The two dealers that have done the work,even replaced the Redline with Redline!
Mak sure you go back and make a point for him to prove himsefl and give him all the opportunity. Its these idiots that are working on our figgin bikes! This is just a shamed. When he can't prove it let him now you can't argue with INORANCE!