I have tried a couple different ones. Cant remember names. I just bought the Marvel Mystery Oil. Is that any good? What worked for you guys? I want to try and get better mpg.

 

Its my opinion that there are no additives that are going to give you better millage [ 1 mpg or less ] .
Beyond that there are a few that are good for certain uses , & some for every fill , but more info would help to tell whats , what .
Go to the user control panel and fill in truck info , then we can give better answers .
Some fuel systems are lubed & cooled by the fuel , so some use 2cycle eng. oil , without knowing what you have , can not suggest anything .
Some bank on the name [ Marvel Mystery ] does not mean anything except that its a high grade machine oil , no mystery or marvel .