A-6/ Fuel quality & catalytic converters

[cnu captains]

We recently purchased a used 03 A-6 with 3.0 engine. Since we also have a Jetta, we ran regular unleaded fuel in the A-6 also. The check engine light came on - the Audi Dealer checked it out. They said to run only unleaded premium (93 octane) in this engine or it would burn up the catalytic converters.

We never ran premium in the Jetta, and had no problems. Any advice??