With only 30k miles covered in 5 years I seriously question whether you would want to change your transmission oil. I suspect it is quite involved and hence not a cheap procedure. My 220d Sport has just turned 5 years old and I have switched to an independent garage with MB diagnostic equipment and the ability to update the digital service record. My A7 service last month was £203 inc. VAT. The owner was an MB service technician for over 20 years before starting out on his own. He said he would be looking to change my transmission oil at the 7-8 year mark given I currently only have 22k on the clock. A 3-year MB service plan is currently showing at £40/month. That would make my A or B services £480/year but, hey , that does include a free AdBlue top-up.