Asked my wife, a violin maker. She says some people talk about Strad's oil varnish being the ne plus ultra, but she does not believe there is any difference, and they did not teach them that there was any difference in violin-making school. If I remember, I'll ask a number of makers I jam with 2d week each month.
If there is ANY difference, I would imagine the difference to be minischule, and - as
Sam observes - far outweighed by any variation in wood quality, carving, etc. Good luck trying to get any 2 instruments which are IDENTICAL in all respects save oil vs spirit varnish!
I'd be interested if you were willing to tell us more about the basses you are looking at, how you feel about their playability, sound - AND appearance.