Snake Oil's are good for one thing and one thing only...
making you feel better. Thats it. If adding Durralube et al into your crank case makes you feel better about driving it, maybe it's worth it. HOWEVER, I would seriously stay away from Slick 50 or anything that takes 10 mins to come out of the bottle (engine honey-esque additives). I'll agree with the Die Hard Techies, just change your oil regularly and you'll be ok.