Originally Posted by tagesk
As a side-effect: If no-one can produce any substantial information on this, pointing to this thread will
enable us to shoot down he 10w-40-advice whenever it pops up in future oil-threads regarding the wethead.
Well let me try. I went thru a series of changes with the K1200S wet clutch bikes. I had an '05, '06, and now an '08. BMW tech bulletins recommended 10W40 if I remember correctly at first. Then there were reports of the wet clutch being starved of oil...clutches squeaked when engaged...and many were replaced. BMW then came out with a modification for an oil passage to be enlarged as well as some warranty authorization for thrust washer tolerance at the back of the clutch basket. Some mechanics were reporting "dry" plates when disassembling the clutch basket and plates. All this was followed by BMW introducing a new viscosity oil which was 5W40 here in the US bottled by Spectro. and publishing a tech bulletin specifying the new 5W40 for the K1200 engines. According to a mechanic friend at a major dealership, the BMW tech rep recommended Castrol Power RS Racing 4T 5W40 full synthetic.
note the JASO-M2 on the label.
I used the BMWNA branded Spectro 5W40 fully synthetic, also JASO-M2, which continues to seem fine...no noises...clutch seems fine...feels smooth, no squeaks, etc. I have about 30K on mine and 20K on my wifes '08 K1200S.
I think BMW has come up with 5W40 as best for their wet clutch applications. And being water cooled, better internal engine dimensional control with temperature, the "lighter" weight is appropriate.
That's my take on it.