Do you add Marvel Mistery Oil to your Engine & Gas Tank-For how long do you leave it in your motor

I’ve never run it in oil but I ran it in fuel steadily in

1. 82 Diesel suburban (better mpgs)
2. 1998 Buick Lesaber failed leaky intake
3. 1970 Subaru 360 (better mpgs)
4. Chrysler minivan (burned lots of oil)
5. 1993 Suburban after it started burning oil

I’ve tested it in good running newer cars and found no significant improvement, it seems to improve fuel economy in old cars though