Marvel mystery oil used as a lead substitute?

4 oz to 10 gallons for fuel. MM was developed for radial aircraft engines in WW2 era. Google it and you can find an article of the development, it's a good read.
I would run what you have and not worry about it. You may have induction hardened and not even know it. The auto industry knew unleaded fuel mandate was coming and didn't wait until 1973.