I think this is block exemption again.
In theory if a car is serviced exactly according to the manufacturer's instructions using approved parts and materials the manufacturer must stand by the warranty. This includes ensuring that the exact grade of oil etc. is used.
In practice there are a couple of considerations - if a car is serviced within the dealer network then there is a possibility of goodwill if a fault occurs out of warranty, this evaporates with non-dealer servicing. Also, "premium" manufacturers such as Audi will not give full trade-in value on non-network maintained vehicles, they will only give auction prices.
It's always worth remembering that you can get the car serviced within the "group" and maintain the network servicing, for example Skoda or Seat dealers will be happy to service VW or Audi cars at much lower prices.
SSE