All the "miracle in a can" solutions (stop leaks, flushes, "stabilizers", stop smoker, etc) are a complete waste of money. The only place they should be acceptable to use is on a beater trying to extend its useful life with minimal cost/repair.

Stop leaks will block many small passages like heater cores, radiators and (for oil) the piston cooling jets, oil cooler and filter.


Flushes are solvents that will also strip off the lubricating film in addition to sludge. VERY bad for the turbo bearings and cylinder walls!
http://autotechrepair.suite101.com/article.cfm/is_engine_flushing_necessary

Stabilizers are thickeners that have lots of various additives that can conflict with the engine oil's additives. They can cause foaming and oil starvation.
http://www.bobistheoilguy.com/images/lucas/lucas.htm

Stop smoker is an oil thickener and raises the oil's combustion temperature. This can cause carbon buildup in the combustion chamber and oil starvation at startup.

The bottom line: If your car has a problem and you care about your car in the least bit, fix it properly. "Miracle in a can" solutions only cause problems in the future.