Re: Oil sludge
Run a good quality oil and filter and change more frequently. We've said that doing a flush can frequently cause more problems than the dirt you're trying to remove. Additives are more designed for a maintained engine to keep it maintained, but as Logan said, today's quality oils make it moot to use an additive. If neglect has caused a poor condition in your engine, the correct answer is to pull the head and oil pan and clean it out.
'Scuse me, while I scrape this Honda outta my treads.