My advice is to ditch the AG window cleaner and buy some Meguiar's NXT window cleaner. It is a far superior product and requires less effort to clean your windows. I would then apply a water-repellent sealant like RainX once your side windows are very clean. The RainX ought to be applied on the exterior surface of the windows only and you need to follow the manufacturers instructions carefully.
The design of the MINI side windows means a pattern of rainwater leaves marks around the front and top edges. RainX ought to reduce this marking quite a bit.
If you want a far better product than RainX then try to get hold of Carlack Glass Sealing kit. It's very hard to find but works better than RainX and lasts much longer. You would only need to clean and seal your windows 2-3 times a year with Carlack. RainX may require 5-6 applications during the year.
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