This same subject - what type of oil to use - comes up repeatedly on every antique/classic car/motorcycle forum I've ever had the pleasure to grace. My $0.02 worth:
- Any oil is better than no oil.
- Clean oil is better than dirty oil.
- If your oil is low, add a quart of whatever you have rather than wait until you have the "right" stuff. See #1.
- Any oil these days, synthetic or dinosaur based, is better oil than what went into these cars when they were new.
- Since we change the oil so often (see #2) it really doesn't make much difference what we use.
- Pick whatever oil you like and can get locally and inexpensively (in my case it's Mobil 1 20-50), carry a couple quarts when you go on tour, check the level frequently and don't sweat it.
JayJay