Fixed or variable is dependant on what oil you engine takes.
Eg diesel with a dpf are on variable usually using low ash oil 507 spec oil
petrols are usually on fixed servicing on 502 spec oil
performance engine like the 2.0 tfsi and s3's golf r's are on different oil still can't remember the spec
basicly if you on variable you can go on fixed servicing but if the oil you engine takes is supposed to be fixed you can't go on to variable. At the end of the day you can choose not to do oil changes on time and stretch it oou but if you have an engine issue warranty will just laugh at you