I have been using Epifanes for well over 15 years and never a problem.
The trouble with 'mineral spirits', is there is no standard as to what they are .
As in all mineral spirits are not created equal. Mineral spirits are for cleaning only .
I use Epifanes thinner when needed and I use plain old steam distilled turps, with no adverse effects. I use Petitt #120 thinner also with no adverse effects.
I avoid all WEST products for the same reasons I don't eat at McDonalds , drink Bud or use AOL. I feel they are all overpriced hype and no value for your good old US dollar .
Epifanes is not cheap , but it works better than all the rest .
If I could still get it , I would use Callahan's Chiltered varnish .
To me, WEST is for people who have Goodyear tires on their Crown Victoria because that is what they came with and the people at Ford must know something they don't know . It's like those little lunchs for kids with the crackers, cheeze and meat all in one little packet . ;>)