If the lifter chamber or the rocker area are clean, then the previous owner likely was using an oil with a full additive package in it. Without detergent/dispersants sludge will show up relatively quickly, especially in an engine used for frequent short use operation. As for "impressive" additive products, I've been exposed to a long list of "miracle in a can" additives over the past 50+ years of automotive operation............on close review they all make essentially the same range of claims, just with a different set of words and graphics. They come.....they go.....the wheel keeps turning. If they don't cause any harm, a buyer is okay to use them........whatever makes them feel good.