I had this on my 600E. Over a few weeks, the seat became more and more sticky. I did a bit of research (now forgotten the details, sorry) and it looks as though it was the resinous deposit from the trees round the house. Nothing -
nothing - would shift it. I tried all kinds of cleaners, solvents like petrol and thinners, WD-40, bleach, everything. All I got was an even muckier mess. As the seat cover was already starting to split, I just replaced it.
Sorry I can't be more positive
