tlak, in my case it did leave residue on the windows, but I used over 1.5 Litres.
Cleaner, your point is an interesting one. Logically, what you say is correct and won't do harm following it. But it may be wasteful of the PC. You see, RO is the
deep cleaner, and PC the
surface cleaner. PC is a diluted version of RO. But RO is also the conditioner. No matter how dirty and clogged the leather is you need to start with RO. This will dissolve all impurites, be absorbed deep into the leather, and displace (pushing from the inside out) all the deeper impurities. They will rise to the surface of the leather and look like a white residue or scum. Then you use the PC to dissolve and clean the surface residue.
This process can be repeated until the leather can't absorb any more and the RO just sits on the surface. Clean with PC, let dry, then buff with a dry towel.
Allowing the car to sit in the sun with closed windows is key to RO working properly.