most shops will tell you to stick with OEM because they don't know nor support aftermarket stuff and don't want to be held reliable some how if they tell you to use aftermarket filter. They will also tell you to use the "replaceable" when you can since they make money with service and 30% on parts - after all they are in what is that called hmmm..... ohh it's a business to make money
I've been using airintakes and K&N filters since 1998 and never had a problem with the product or an engine. Can you over oil the KN product, yep I've done it on my GTO. If you drain the son of a bitch of course the sensor will get f'ed up the extra oil will be sucked toward the mass air sensor <-- this is simple science folks.
Now, using the KN product on a BMW different? Well, I've seen BMW road race teams with K&N filters on 3 and 5 series rides, some are mostly stock just bolt on stuff and some are fully gutted running straight race fuel. The moral of the story is IMO is this, K&N will not produce and sell a product successfully over 30+ years if there is a problem with a huge number of people.