I thin with humbrols own thinner.... sepecially when finishing. I some times use cheaper turpenoids for base coating and filters but for a guaranteed nice finish I would use their own.
But to answer your question ...... if it works for the rest, it should work for the varnish. here in sweden where they use all types of things for thinning, if it is coloured thinner, this will effect the finish. Also beware that varnish can darken the finish!