the detergent compounds are what cause the swelling of the rubber used to bind the cork together. they attack all kinds of organic materials.
I've tried all different kinds of oil in tomos transmissions, anything motor oil is about the same in terms of shift/slip. its weird on an e-50 you can tell the difference between 10w30 and 10w40, but in a tomos unless you switch from atf- motor oil- gear oil its pretty hard to tell.
I like to use 10w40 made specifically for motorcycles, in the US you can buy it from valvoline at just about any auto parts store. i'm sure someone in the UK is selling it. It has the proper additives for wet clutches. Been using that stuff in ZA50's and A3/35's for years and it seems to work really well. I don't get any oil-related clutch material failures.