Marvel mystery oil used as a lead substitute?

Used MMO for many years when I’d run higher compression motors. MMO is a lubricant so I can’t see why it would be a detriment to a motor. However Seafoam can dry out seals if used continuously. I’ve taken apart carbs where Seafoam was used. Seals pretty much dried out.on the accelerator pump on a Holley the umbrella seal would be totally eaten away. On
Y problem I found with MMO I would get a reddish stain on my rear bumper where the exhaust exited. So, I guess you have to pick the poison you are comfortable with.