On a mirror test what are you looking for? A residue left behind?
The deposit that you are looking for is oleagenous waxes, commonly know as paraffin. It is non toxic enough to seal jelly jars with, but clogs expensive lighters. Check the MSDS.
It can also be removed along with the plant waxes by winterizing, if you are concerned.
The mirror test is old school and old news and not as accurate as buying a product from a reliable manufacturer, that has been tested by an independent testing lab and certified.