Think about it. How do you dissolve/loosen/remove laquers, varnishes, gums, etc.? Strong solvents right? What do strong solvents do to oils and greases? If you do succeed in dissolving sludge, varnish, gum, etc., where does it go? Circulates in the oil and gets trapped in the filter? You hope. Does it pass over/through wear surfaces or friction points on the way to the filter? How do you know? Does any engine manufacturer recommend using motor flushes and the like? Have you read your owners handbook? If so, what does it say?
My best option is to follow the manufacturers recommended service schedule using the recommended products. Probably your best option too. If the owners handbook says use API SL rated (for example) motor oil in 10-30 grade for your type of service, then why would you do anything else? If it says to "flush" the engine with XYZ every x number of miles (bet it doesn't) then that's what you should do.
Like the guy before me implied, if it ain't broke; don't fix it.
Short answer: "flushing" an engine out with some kind of solvent product in addition to the oil is silly.
My $0.02
