If it has this one problem that you know of, it is very likely -- I'd say 99 percent certain -- that there are other problems you do not know about. Problems are due to age, and age affects everything on those things. Mostly likely the oil inside that dial has hardened over the last 50 years, it needs to be cleaned and lubed.
But there's oil all around that camera, every bit as old. Just because something seems to be working today doesn't mean it is working right, nor that it will be working in 6 months.
If it is a 3.5 planar, does it have a removable hood? If so it is an F model. If not, probably an E. Either way, worth minimum of $700. Dunno what you paid, that doesn't matter.
It is imminently worth repair, and should be if you intend to use it for anything other than collecting dust. A full service by Harry Fleenor will run you probably $300 or so, maybe more, hard to say. But you will then have a Rolleiflex that works like new and could not be replaced by a new camera for anything less than $4,000. Or more.
Get it serviced. Don't dink around with it. Putting oil into the gears around the shutter dial is asking for trouble, not to mention a much higher bill from Harry when he has to clean up your work.
