This, this, this.
Unless you know you have sludge I wouldn't dump anything in your engine except a quality synthetic motor oil on a decent OCI. And even if you have a little then I'd do the same. But unless you look under that cover you're never going to know one way or another, there's no telltale sign unless the thing is clogged with sludge. Then (so it is said) you get excessive consumption and white smoke from the tailpipe.
Originally Posted By: A_Harman
I liked the suggestion of taking the car to a trustworthy mechanic and having him pull the valve cover. Once you know what the inside of the engine looks like, you can decide if an engine flushing solvent is necessary. I mostly don't trust them, and prefer gradual cleaning with an oil like Mobil 1 High Mileage, Pennzoil Platinum, or Pennzoil Ultra.
I think you should start changing your own oil. It gives you a chance to look at its mechanicals and possibly head off other things that may be about to go wrong.