20k? Just run in, that is.
I've had three K1300S that have gone beyond 38k on the original radiator, so they don't fall apart instantly.
It takes winter riding + no annual (external) radiator cleaning to clog them. That or bad luck. Who knows.
A hint of 'winter riding + no cleaning' can be identified by peeling shaft drive and front wheel holder paint.
They all bubble eventually, but if it's dropping off rather than bubbling then consider a price drop or another bike. Just bubbling - yup, that's par for the course.
The current radiator (39k) is definitely sub-optimal in looks, but it still works.
New ones are around £500 mark plus labour.