In the mid 1980s, I was friends with a dentist who explained it to me this way: The growth of the microbes that cause tooth decay depends on (1) tiny bits of food stuck on, or in between, your teeth, and (2) a favorable pH level in your mouth. Chewing some gum immediately after a meal helps get rid of the food particles, and stimulates your saliva production which alters the pH. The question of whether you chew sugarless gum or regular gum doesn’t make a significant difference because the sugar is pretty much gone within the first minute of chewing and the pH is more important anyway. So, according to my dentist friend, the truth is that 4 out of 5 dentists recommend sugarless gum and the other 1 out of 5 say gum is gum and it doesn’t matter what kind you chew.
OTOH, if you make a habit of popping a stick of gum into your mouth because you want the sugar, and then sixty seconds later spit that one out and pop a fresh stick in, lather rinse repeat until you’ve chewed 60 sticks in one hour, then in that situation you’d certainly be better off with sugarless gum. But that’s not how most people do it.