I think you'd be fine with either. Gun parts move relatively slowly in terms of surface feet per minute and the loading is not extreme. Both are old technology lube viscosity enhancers, not necessarily designed to be lubricants themselves.
Frankly, I'm surprised either is still available. U.S. car mfrs are pretty adament about not using such additives today. And with 100,000 engine warranties, why would you? It's well established that thin oil prevents wear when the oil is cold better than thick oil and that's when the most wear happens during a usage cycle. That's why GM recommends a 5-30w oil.
For a light oil, I think you'll not find much better than one of the synthetic motor oils.
