Hermann from HVDA says in his instructional manual says under <span style="font-weight: bold">"no circumstances use synthetic oils"</span> His recommendations below...
Redline Gear oil, 75W
Valvoline GL5, 80-90W
Kendall Gear Oil, 75W
API Gear Oil GL4 or GL5
(recommended oil viscosity SAE 75W-90 or 80W-90)
There has to be a reason Hermann is recommending this. I'll abide by his expertise. I'm using the Valvoline GL5, 80-90W.
Call him up and ask him why he doesn't recommend synthetic.