IMO, never put ANYTHING but the recommended oil in your crankcase, and the minimum recommended octane fuel in your fuel tank (well, maybe I'll make an exception for fuel stabilizer as it's a necessary evil for those that store our cars).

Basically, oil additives contain detergents and lower grade motor oils that can lead to sludge, increase windage losses, retain certain chemicals, or lead to deposits. Back to the oil sludge problem. Synthetics are great because they don't easily degrade, and have high thermal capacity. Adding some oil additive could degrade that thermal performance and lead to sludge (which is thick goo, and in a nutshell oil sludge destroys engines). A lot of higher performance engines use synthetic because it is able to cope with extreme temperature variation in such cases as a high position mini-cat, hot exhaust manifold/header as the result of retarded ignition timing, etc. You read that last part right - advanced ignition timing causes temperatures to increase in the combustion chamber, and retarded timing causes exhaust temperatures to rise. A lot of cars (even high performance ones) will retard the ignition on start up to heat up the catalyst sooner, as it must be hot to work properly. Once it is warm, the ignition timing will be advanced per the mapping in the ECU.

Fuel additives often fix one problem, but cause another. They often contain compounds that do not melt or degrade, sometimes things such as MMT (a manganese compound) or Ferocene (an iron compound) are used as octane boosters. These ALWAYS lead to deposits over time, which cannot be removed. Combustion chamber deposits (such as carbon, metallic compound build-up such as ferocene or MMT, etc) can lead to problems such as plug fouling (misfire code). If thick enough, they can cause burned valves (from coating the valves, which degrades their ability to dissipate heat). Sometimes, build up of deposit can cause local hot spots, which can cause pre-ignition. Pre-ignition, of course, kills engines.

If it's not recommended in the owner's manual, please don't use it. Or, if you've GOT to use it to satisfy some deep emotional need, please check with us first, okay? We'll talk you out of it.