Id try a few other things before touching any wal-mart additive proclaimed to clean sludge.
Id start by considering a synthetic oil with high detergency... a synthetic diesel oil would be best.
Since that is pricey, Id instead do a few fast changes with a HDEO.
If that isnt strong enough, Id take a quart of oil out and put a quart of MMO in.
If I wasnt opposed to mail-order solutions, Id buy Auto-RX or LC20 and use them as directed to clean out.
If I was sure that I had sludge, Id pull the valve cover and the oil pan - since typically neither is all that tough (depending on engine type, of course), and manually and very carefully try to remove everything that I could.