When I first got my current car I changed intakes and the engine was completely clean so i am running "ZMAX" and I can't say its hurting anything but I can't say its helping either, but the engine seems to turn over easier.
In another car I used to use "STP" oil treatment but if your valve seals and guides are really worn it will make your engine smoke.
I really don't see a "mechanic in a can" as working.
But back to the oil deal, the high priced oils are all synthetic and if your engine is older and you have been using regular dinosaur oil then expect a leak in the future.
So the best you can do is just find an additive and hope for the best.
My recommendation is "Comp Cams" additive.
I have not tried "Lucas" oil additive but did use the fuel injector stuff and now my 01 Dodge truck wont start on the first turn of the key and it takes two times and a long period before it starts, and it all started with that tank full of gas with Lucas injector cleaner.