Straight from Barnett:
In regards to the primary oil, we do not recommend using the HD Formula Plus (the regular old HD primary oil is fine). Any of the aftermarket brands such as Motul, Drag Specialties, Redline, etc. are all fine to use. ATF (Type F) is also ok- B&M Trick Shift has been used for many years with good results as well. If you go with Mobil 1, just make sure it is their oil designed for use in the HD primary (not automotive). The main thing is to just keep good light weight, clean oil in it so it can do it's job to help cool the plates.