I have read shit loads of forum posts on primary oil recommendations for Barnett clutches.
Hot weather v cold weather, synthetic v mineral, not forgetting, to use ATF or not.
Barnett recommends on my paper work ... same as later documentation, not so with pre 07
1) fill with correct type of primary oil
2) do not use formula +
3) recommends type f atf
Variables included, but not limited to ... hp/tq outputs, HD model, tranny type, riding style and experience, viscosity grade.
Open for discussion and debate, here is a link on viscosity .... http://www.bobistheoilguy.com/viscosity-charts/