Oil additives is not suposed to be added to your car...
it is mentioned in the owner's manual.
I heard many horor story of blown head gasket, or seized piston or excessive residue buildup when using oil additives.
As long as
- you are using good oil (synthetic or conventional are ok.)
- change your oil religiously, synthetic is OK for a bit longer interval
You are in good shape. Why adding those thing and void your oem warranty?
But, fuel injector cleaner is on the other hand, works (do help).
Buy hi quality ones like techrone super concenrated ($8 per bottle). Use them sparingly, maybe every 8K miles or so.
EPA has done tests on all of these based on their claims to extend fuel economy. NONE of them work as advertised. Fuel economy is improved, at best, a statistically insignificant 2%. Do not bother with the oil additives. As SRM said, likely to do more harm than good.
Gas additives are a slightly different story. For example, Techron brand carb/fuel injector cleaner (made by Chevron) does work. I have it in my motorcycle and it cleans out the carbs pretty well. It won't unclog gummed up injectors (its not THAT good), but it will help clean off small deposits and prevent build-up. Its intended to be used periodically - not with every tank of gas.
I agree with the Techron fans. I use a bottle of Techron concentrate every 10,000 miles, and it has been proven to clean deposits from injectors and valves.
As for the miracle oils, if you do a Google search on each of them, you will find most have settled in class action lawsuits due to false advertising. I do not believe they work, although I suppose if I had an old beater, I'd be more willing to spend $10 to try a miracle oil before spending $1000 for a major repair.