Well, every 3,000 miles or so I usualy dump about 1/4 cup of Marvel Mystery oil into my gas tank right before filling it. I've been doing it for 4 years on the 5.0 now with no ill effect's, and it definatly helps to de-crud the fuel lines / injector's. I usualy notice a bit more pep at the pedal when I use this. I also used to use it in the crankcase, but Ive switched to synthetic oil and Im a bit nervous about adding stuff to synth. (Just my own uncertainty there).
If you want to add some to your oil, best bet would be doing so about 300-400 miles prior to an oil change, give it some time to work at any of the crud already existing.
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