First off, never add anything to the oil or gas "just because."
Unless your engine has serious problems, just about any additive will probably do more harm than good. Almost all the additives I've seen are meant for worn out engines that you want to get a few more miles out of before rebuilding/scrapping or they are snake oil. For extreme conditions (eg, supercharging, racing), there are some additives that can help but I don't think you're going to find them for $1 unless they are off-spec/out-of-date/stolen/&c.
There are some additives that will help specific problems but those should be used on an as-needed basis - if it ain't broke, don't fix it. Assuming regular oil/filter changes and good fuel, you shouldn't have to add anything (other than the rare dose of injector cleaner) to the gas or oil.
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