Masturbating a penis for a long time with a dry hand can cause abrasions, and the skin on a penis takes the longest of any skin to heal. This means that you’ll basically get a scrape on there that can potentially last for a week or more, and you’ll be reopening that wound every time you cause dry friction against the penis skin. Wounds like that can cause sexual dysfunction in the future by interfering with the penis’s ability to experience pleasurable sensations, since it’ll be painful at those spots instead.

So if you’re planning to have long sessions, or many sessions in a say, use some kind of lubricant. Lotion should be fine, but seriously formulated sexual lubricants are even better since not only do they provide lubrication for your safety, but they can increase pleasure by simulating the wetness of an aroused vagina.