Just purchased a 2002 Audi A6 27 from a dealer. Ssalesman never mentioned the use of premium grade fuel. Owners manual states use premium octane 91, but also says can use octane 87with a certain loss of engine power.. Does using 87 at all detrimental to emissions or catyliter converter or other parts.
What do you recommend?Premium gas is expensive.

I would use 91 octane. Many Tier 1 premium gasolines (e.g. Shell and Chevron) have more detergents in them. This will pay off, in the long run, with cleaner injectors, valves, and pistons. It will also help prevent damaging engine knock. Most high performance (high compression ratio) European engines should be run on high octane gasoline. It goes with the turf.