Don't worry about your warranty. I run an independent garage and we've been servicing cars from new since the block exemption laws came in. We always use genuine parts while cars are under warranty although strictly speaking you don't have to, they just need to be of equivalent quality. We've serviced plenty of cars that have been back for warranty work and it's never been an issue. I wouldn't say they like it and BMW have made us send all sorts proving the quality of the oil and service was done correctly before now, but at the end of the day the law is black and white. The vehicle must be serviced in accordance with the manufacturers schedule using parts of the same quality. Just make sure you get a vat receipt and that they have the diagnostic equipment to check and clear any fault codes. Then laugh your way home having saved about £100