Your siding will chalk and etch over time and that's responsible for the dull look. Really, it sounds like it has held up pretty well after 20 years exposure to acid rain and other things that have gotten on it. The solution to bringing back the look is paint. I suggest that you go to your friendly local paint dealer, instead of a big box home center, and work with them. Local paint stores, like Sherwin-Williams, Benjamin Moore and other first quality paint retailers, tend to offer a lot of expertise to their customers. My experience has been that their good information and the product quality more than outweighs the slightly lower cost of home center paint.