Does 2002 vw passat v6 engine need 91octane fuel?
Yes. It's a higher compression engine which requires a higher
octane level to help prevent "pinging" in the engine. This engine
does have the intelligence built-in that detects "pinging" and
retards the engine timing to prevent to pinging from occuring.
Pinging is when the gas explodes early while the piston is still
going up. This can cause damage to the piston over time, something
that's expensive to fix.
If you put 87 octane in the tank the car will detect the pinging
early, retard the timing to save the engine but the fuel economy
and performance goes down. Generally speaking you'll pay more in
gas by dumping 87 in the tank than filling it up with the good
stuff since the performance is better.