In a nut shell, Auto-Rx works best in both parts of the application, cleaning and rinse, with a conventional or group III oil. Once you get to full synthetic oils, called group IV, the oil contains a high level of esters for coupling in the additives to the PAO base. It is desireable for Auto-Rx to be run in an oil where there are no esters, other than the Auto-Rx itself. Ester are polar in nature, which for our purposes means and attraction to metal. The esters in Auto-Rx although great lubricants in their own right, have great cleaning ability. The esters in the group IV oils are not very good cleaners at all and exist only to help the couple additives into the oil base, but still have an attraction to metal. So you have Auto-Rx esters and group IV oil esters competing for the same space. There for the group IV oil will not your best choice for oil in cleaning.
In the rinse phase, ideally you have no esters at all. That is why we rinse with no Auto-Rx installed and recommend a conventional or group III oil. You can rinse much better with no polar ingredients, or ingredients that are attracted to the metal. Simple mineral based oil works best for rinsing.
