We have the same problem with the stiff track of a 25 year old sliding patio door, and candle wax rubbed over it can help ease it. You can use an ordinary unlit candle, or in your case with the amount of doors you have, melt a candle or two and brush the liquid candle wax over it. Another alternative would be to get a tin of beexwax polish and a soft cloth and rub a generous quantity of the track rails. I'm not sure whether a light coating of oil would also work. You could try one set of doors and see if it's effective. If you have a centrally heated house, it may be that any natural oil which was there originally has long dried out.