First let me state that Auto-Rx works very well with Pennzoil Platinum for cleaning and rinsing. However, Platinum is a group III mineral based oil product, highly refined. On long oil change intervals it will darken significantly and will have some volatility burn off, although much much less than a group II conventional oil.
With regards to staining in aluminum, sometimes the actual brown staining can be into the aluminum, as opposed to merely a coating on top of the aluminum surface. Auto-Rx will work well in areas of forced circulation on these stains. However, in more cosmetic areas, where there is only an occasional splash or no oil flow at all, the stains will remain. I am quite certain that you would be happy with the Auto-Rx cleaning performance with the Platinum oil.
Varnish typically forms from the oil and fuel vapors that are trapped under the valve cover each time you shut off the motor. You might want to check or replace the PCV valve or other breathing apparatus to be sure that a good majority of these vapors are being recycled through the intake, as opposed to being trapped internally.
Varnish stains on areas of the motor that don’t involve working parts really represent no danger to engine life.
