From my days in the European aftermarket world, dealing with an Audi ECU tuner based in California (and thus tuning on CA's 91 oct), the average comment was that CA's 91 octane was in actuality nearly identical (from a performance/knock standpoint) to the East coast's 89 octane. Assuming this is correct, it would roughly equate to 93RON. Again, assuming the tuned RS6 was meant to run on 98RON, I could definitely see where the disaster potential exists. There was a point in time where that tuner was going East with a truck and 10-15 5 gallon fuel cans to fill with 93 oct to get proper "rest of the country" tunes done