I inherited some older stones from my dad in a cedar box. If I remember right he added an oil to them back in the day. Is the oil needed or can they go dry ?

Oil needed when sharpening.

3 in 1 oil is fine, but a thinner oil might be better. That said, I use the 3 in 1. Mostly I’ve gotten away from stones, and use diamond plates these days. The only stone I use is a black hard Arkansas stone that use last to polish up the edge a bit before I go to the lather strop and jewelers rouge.

Just to mention it, you can buy a plate to flatten and resurface old Oil stones. Years of use of oil stones could lead to the surface being loaded up to the point that they have little actual cutting surface working. And they may need minor flattening.

Old stones are still good. But, as stated, they need to be cleaned to remove accumulated grit. Easy way soak in water and then scrub them off with some kind of brush.