We use a combination of German, Spain, and USA made varnish, spirit and oil, or mix of both.
In general the oil varnish takes much longer to dry, and can produce a richer, complex tone. Though both oil and spirit varnish come in different grade, a higher end spirit varnish is definitely better than lower end oil varnish, as some Cremonese makers are for sure using spirit varnish and have achieved ideal appearance, and promising tonal result.
One extremely important aspect in varnish, is the ground coat, which seals the wood and has a huge impact on resonance. Testing and experiment is necessary for ground coat formula, since different tonewood will have different response to same ground coat/varnish; while too thick the coat will suppress the vibration, and too thin will make the tone overly aggressive.