I was a member of the z3 forum for years and the universally recommended rubber conditioner was something called "Gummi-pflege" (could be slightly wrong spelling)obviously German and it was remarkably good at refurbing/moisturizing the window surround rubbers which used to age and shrink which caused small gaps so you would get problems with condensation and creaking hardtops.
Its probably available on fleabay and I think was sold at bmw dealers at one point.
Gummi-Pflege (82 149 407 015) - which translates roughly to "rubber maintenance" - is BMW's solution for exactly that.
Its been a staple in the Z3 community where the rubber seal that runs between canvas top and window also encloses a water drain channel. To accommodate the folding of the top, the seal is split in a couple places along the window. When the rubber dries or becomes weathered, it tends to shrink. When this occurs in the Z3 window seal, the water draining in the channel leaks at the splits, and the driver develops a nice puddle on his/her lap.
Gummi-Pflege helps recondition older seals and reduce or stop water intrusion in typical problem areas such as the trunk and window seal. It certainly makes more sense to spend £7 and a little time trying to recondition a leaking seal or one that propagates wind noise before looking to replace, say, a £300 door seal.