I guess I'm one of those old codgers, as I remember when lead was pulled. For a little while, you could get both (I think the three pump choices were premium, regular, and unleaded). Then it was gone altogether.
I'm glad it's gone, but it was a bit of a problem on some of the pre-1970 engines when it happened. Back then, we would use mmo and a few other "top end" lubes in the fuel more for valve seating and lubrication than anything else. It helped, a little. It was especially difficult when lower octane unleaded fuels arrived. Then we had to "lube the fuel", retard timing curves and or fuss with things like water injection to keep 'em running decent . . . or hunt for race fuels on weekends.
I'll still recommend mmo for vintage and antique engines still stock, especially with today's alcohol-laden fuels. It also helps, a little, with the older carbs.
Not sure that mmo is really needed on today's engines and fuel delivery systems, even with the alcohol. There, I think a more modern detergent chemistry like techron can do more overall good than the mid-century chemicals such as mmo or sf.
I honestly don't know why anyone would add either of these to a crankcase for PM. Their reduced levels of lubricity and increased solvency compared to a good motor oil isn't the best tradeoff. I know people do, though.