MACK said:
I use the autogylm cleaner and sponge but dont find their water proofer lasts too long. So I use Fabsil Gold to water proof it. It was originally recommended to me by a car detailer. It isn't a car product as such and it's available in places like Go Outdoors. Its designed to water proof hiking clothes/boots and tents etc but works well on car roofs and last ages if you dont use strong cleaners on the roof (which your not supposed to do anyway)
If you do use it make sure you get the gold stuff, its a bit pricey but is something like 5 times as strong as the regular stuff.
You might want to check out the chemistry in Fabsil and make up of the BMW triple layer roof. The top layer is not supposed to be totally 'waterproof' so that the material can dry fully without being trapped, many people use other water-proofers and swear by them but that is not how the is designed to work (the company that make BMW roof materials also do Audi, Mercedes etc).
Fabsil is nearly 100% Heavy Petrolium Naptha. The actual waterproof layer underneath is made of Butyl. According to freely available technical data the resistance of Butyl to Naptha is the lowest rating at 'unsatisfactory' i.e. may be damaged by Naptha products.
Fabsil data sheet http://www.farnell.com/datasheets/1536436.pdf
Rubber resistance to chemicals http://mykin.com/rubber-chemical-resistance-chart-4
Previous post discussing these topics http://www.z4-forum.com/forum/viewtopic.php?f=15&t=81422&hilit
I stick to BMW cleaner and treatments, at about £12-£16 a time they have lasted for 1-2 uses a year for about 6 years and only just run out, but other members get good results with other brands (which are probably cheaper and made by the same people

)
BMW stuff http://www.bmw.co.uk/en_GB/topics/discover-bmw/accessories/accessoryDetail.accessoryId=246.html#eyJhc19xIjoiaW5tZXRhOnByb2R1Y3RDYXRlZ29yeSUzRE1hbnVmYWN0dXJlckFjY2Vzc29yeUNhdGVnb3J5NyIsImFzX29xIjoiaW5tZXRhOnByb2R1Y3RHcm91cHMlM0RNYW51ZmFjdHVyZXJBY2Nlc3NvcnlQcm9kdWN0R3JvdXAxOTQifQ==
BTW I use Fabsil gold on outdoor kit and really works