I have a 1998 A6 with 80k miles. I just changed the spark plugs and was surprised to find VW NGK plugs in it.
I put in Bosch Platinum Plus-4s, $5.99 per plug. Just driving around the block I could notice a major difference. It was not very difficult. Let me know if you want the steps listed out.

 

You know I always thought I really liked Bosch plugs until I realized that the +4's I kept in my 99 2.8 were giving me consistently bad gas mileage. A swap back to NGKs brought the miles back to the mid-22's and I had power again! Your experience may vary, but that's my story!

 

Thanks for the info. So far everything seems to be running well and the gas milage on the dashboard display is normal, but I think it is good to track the actual gas milage.