evaluating & fixing finish on old oak chairs

Hello. First, let me thank all the posters and the experts here. I have learned and continue to learn alot just by reading through the various questions and answers.

I am still working on these two old oak kitchen chairs. They are NOT valuable pieces so they seemed like a good bet for this inexperienced restorer. They each had padded vintage naugahyde seats (not drop in) tacked on with brass upholsery tacks--I removed these not knowing what to expect beneath--it turned out to be your garden variety chair seat. Just an oak veneer but it is several shades darker of course than that area of the seat that was not covered by the padded area. I think the chair seats are too battered with tack holes and (hundreds!) of rusty old staple holes so I guess I need to re-apply a chair pad and i now have some lovely fabric to do it.

I cleaned all the dirt and grim off with Murphy's Oil Soap and some 0000 steel wool. There are a few places where it seems like the finish is breaking down--a bit blotchy and lighter than the stable areas. Given that I don't wnat this project to cost a fortune, is there something that I can apply to the exposed wood that will bring it along. The chairs are plain jane nothing special--the very worn look doesn't seem to detract too much but I would like to prevent further deterioration.