You are before the roller cam era so I would use a 10W40 conventional motorcycle oil for break in or valvoline racing oil in 10w40 after break in, then a 20W50 mtorcycle oil will work well.
They know motorcycle will sit around a lot, and most Harleys still use cast iron bores so they have good corrosion and rust prevention.
Thanks. I do run one of my very limited use cars on M1 20w-50 for those reasons, and it is a recommended weight. I may look towards a non-synth MC oil for this.
MCompact, thanks. I used to run Castrol 5w-50 (before the Castrol vs. Mobil what is 'synth' issue) in my 318ti...miss that car.