Motorcycle oil is developed so it works well with a "wet clutch", that is, one that is lubricated by the same oil as the engine.
Cars usually have dry clutches. If you owned a dry clutch BMW bike (such as any boxer or older K series) it would be perfectly ok to use car oil, but with a "normal" motorcycle, specialy a high performing one, that´s a no-no.
I use 15w50 synthetic motorcycle oil (that´s the oil recommended by the KTM dealer i do my maintenance at).
My SD uses maybe like 100 ml every 1000 kms or so.