In theory a saddle that fits well shouldn’t need anything other than a thin pad to keep the panels clean.
However, for an older horse, especially one that’s lost some of its natural padding, may benefit from a pad with some extra cushioning according to saddle fitter Gary Severson. That was his prescription for Fortune, a 19 year old Hannovarian/TB cross at our barn.
There are several types of pads that can offer protection from concussion. I use a sheepskin or Thinline pad under my saddles or, if the saddle needs a bit of shimming a Mattes correction pad. If you don’t have one of those, a thick fleece saddle pad will do the trick.
Just remember that some thicker pads can affect saddle fit, so make sure you’re not creating pressure points with whichever pad you use.