I got that from a wood workers site explaining velvit oil contents
mineral spirits dilutes the dryers.i thought mineral spirits had a high flash point.
Adding mineral spirits would dilute all of the ingredients equally mc. The Marine Spar Varnish/Tung Oil finish you bought almost certainly had some type of metallic dryer added to the formula. The ratio of dryer to resins and oils would still be exactly the same. The more volatile mineral spirits would flash off or evaporate fairly quick, leaving the lesser amount of finish behind. The thinned coats would appear to dry more quickly only because the material is diluted. Many so-called sealers are nothing more than some type of finish that has been thinned with a solvent to permit deeper penetration, with little or no surface build.
Whether the Marine Spar Varnish/Tung Oil or the Ace Hardware Spar Varnish/Tung Oil actually contained much if any real traditional Spar Varnish or real Tung Oil is another story entirely. Manufacturers have been using modern polymer substitutes for costlier products like Copal Resin, Rosin, etc., and pure Tung oil, for many years. The real things are still available, but are more costly low volume products. It sounds like Velvet Oil is very similar in composition to Watco Danish Oil, which Mike Hunter tells us is what the Winchester Custom Shop uses when a customer asks for an oil finish on Model 21's and Model 70's.
There is a link here for Velvit Products, but the website doesn't seem to be active:
https://www.loghome.com/companies/velvit-products/