BMW used to say it was fine with 20/50 mineral oil about 11 years ago, then they changed their minds after a lot of engine problems and changed to power 1 castrol expensive, ( I used this for a while but when hot the engine rattled really badly)
I then used 10/40 semi synthetic for a while as I've been using it in my tls's for 20 years without problems,
now they seem to recommend 15/50
This came from an ex BMW chief mechanic from wollastons who serviced my bike earlier this year,
Anyway I bought silkolene 15/50 semi synthetic which he was happy to put in it whilst he was sorting out an oil leak from my front engine cover.the engine sounds quieter and feels smoother with this in it,
I can concur with the engines sounding like the proverbial bag of spanners when hot, when using lower or thinner multigrades
xx/50 certainly quietens them down when hot
I see you can get a 10/50 semi synth now might not be a bad choice to cover both ends of the spectrum