BMW Gummi-Phledge

Non shiny, make seals look like new, stops squeaks, stops seals freezing, can cure leaks (from dry seals) and cheap as chips

Its what they recommend for the cabriolet seals and the seals on the pillar-less doors.

A tube will last you ages.

If your BM dealer looks at you with a blank expression (they often do) when you ask for it, go on any BM forum and do a search on the name, then go back to them with the part number …