Since runners tend to be outside more, if you run during daylight hours, you definitely need to wear sunscreen. However there is valid, real concern sunblock can interfere with your body’s ability to sweat (and release heat) and may even block pores. It can also get in your eyes when you sweat, causing a lot of pain, especially for runners wearing contact lenses. And there is almost no way to fix this when running.
For these reasons, I choose not to wear any sunblock. Instead, I schedule my runs to avoid sun by going out early in the morning before sunrise, or in the evening close to or after sunset. This saves time, eliminates concerns about trapping heat, it’s usually much cooler, and there is typically less traffic and air pollution.
But if you do run outside in the sun, experts recommend SPF-30 or higher sunscreen for proper protection. Runners also need sunblock that is sweat resistant. This means the sunscreen is less likely to become diluted (and weaker) when exposed to sweat or water. Many runners I talk to use Neutrogena. They make an “active breathable sunblock” that works well for running.