Instead: Hydrate with H2O. One of the best measures you can take to avoid kidney stones is to drink plenty of water, leading you to urinate a lot. So, be sure to keep well hydrated, especially when engaging in exercise or activities that cause a lot of sweating. It's Not Just the Oxalate.
You didn't say what year or model BMW. The service intervals vary a huge amount between models a years. BMW built since the mid-1980s have a service indicator. You should change the oil when the service indicator tells you to. On BMWs that specify synthetic oil, you should change the oil when the indicator counts down to zero or once a year whichever comes first.
Depends from car to car. Mainly due to the location of the oil filter. But, an oil change should not ever be an oil change. With better oils on the market some cars do not need oil changes that frequently anymore. The average interval is around 8-10k or six months. Toyota for example recommends a service interval of 8k or six months.