Mark, 15W, are you serious? They will be like iron and not return at all.
I like silkolene RSF5, but something around 5 or 7w designation is OK. Idea with cartridge forks like that is you keep the oil the same and change the valving if you need to change behaviour.
http://www.peterverdonedesigns.com/i...osity-Data.gif
here is table. pick one around 25cSt