1977 320i - leaded versus unleaded gas

I just bought a very nice 1977 320i, 100k miles, one-owner family (went from dad to daughter) and very well taken care of. They said they used a lead additive with every (well most) fillups because the engine was designed for leaded gas. Is this necessary? What happens if I just use unleaded from here on out?

It's an old wives tale- the seats in your head are hardened and you'll have no problems.

Richard

Thanks for responding - appreciate it!