Depends what you're looking at the product to do? BG makes a great line of oil additives that help keep your engine running great.

Another that I'm a big fan of is Auto-Rx which is a cleaner for your engine. It removes sludge and varnish from your engine slowly over a few thousand miles. Unlike harsh solvents it works over time to slowly remove the garbage coating the inside of your engine including cleaning up the ring packs of baked on carbon. You pour a bottle or two in when you change the oil and drive your car normally for 4-5K miles and do your normal oil change.

There are a number of before and after videos people have put up on YouTube if you're interested in using it. It is available on Amazon or through the manufacture.

There isn't a additive out there that is going to fix a worn out engine.

All that stuff is snake oil, nothing can fill space if metal is worn out

Your best off using a quality old and and filter and not any of that stuff. Its okay to use on much older cars if needed but anything modern day it's a complete waste of money. 

Some people swear by it and some people think it snake oil stuff .

I use molylube ... I could make all these claims but at the end of the day I feel safe with it in my engine.

My point try and and see if it works.

It been on the shelf for x years so it must be doing something right.

Its a good lubricant...but isn't going to fix damaged or worn out parts.

Worked at auto parts store in the '70s, we sold "TransX" transmission fluid additive by the case to local garages. Main ingredient was kerosene !