[prolepunk]

I think you mixed up the units here.

In us oil changes are 3k-5k miles. which is ~5k km to ~8k km whereas in Europe it's 10k to 20k.

I find 8k-10k figure to be close enough and realistic.

5k km for old cars with non-synthetic oil.

20k km I highly suspect is a pipe dream from BMW marketing material ensuring that after about 120k the car would have no resale value.