Borg Warner Overdrive in a ‘31 Model A

[Mer]

I’ve trying to figure out what fluid to use in my transmission, banjo and Borg Warner Ovedrive. I understand that I can’t mix fluids as the three share oil passages. The trans and banjo call for 600 W oil while the Borg Warner OD manual recommends GL1 (straight mineral oil). Afraid of using GL1 in the trans and rear and concerned that the 600 W in the Borg Warner OD will be too thick and hinder or make inoperable? What to do? What are others using?