Ok. The first oil thread on the new K bikes here...heh.
BMW has said from the beginning on the '05 K12S not to use 20W anything. Only 10W40 or 15W40. Then with the '08 book, they recommend 10WX.
I kind of wanted to use BMW 15W50, but have reconsidered and think I will stick with the 10W40 BMW or Amsoil. Both are JASO-MA good for wet clutches.
I have heard of a couple of actual dry plates in the clutch pack upon dissassembly. And some dealers modifying the oil holes in the clutch basket to allow more oil to pass thru. Not sure about that of course, but it makes me want to stick with the 10W40 all year around.