Douglas:
Cuprinol is a copper naphthenate (organo copper salt) oil based solution. The copper salt kills fungus and is used to prevent wood rot. Copper salts are systemic poisons, hence, as such, are very toxic. The oil based solution has a green cast to it. The color is similar to pressure treated pine. I would not recommend cuprinol for interior use nor mixing with paints or varnishes.
Penetrol is a paint/varnish diluent that cross-links when it cures, hence it is not an evaporating solvent. In addition, it makes paints/varnish flow better. As Ben does, we add a small amount of penetrol to our paints and varnishes to improve flow, especially on hot days. One of our boat friends uses the following schedule for interior wood - two coats of penetrol followed by two coats of satin varnish. The result look great, especially on Australian lace wood. Penetrol contains no UV inhibitors. It also can be used to seal metal surfaces - bronze, brass, aluminum, etc.
Rod