Oh, I wish I lived next door so I could help! This is a project I could sink my teeth into.
Without seeing the chairs, it's hard to say what will give a finish you'd be pleased with. Trying orange or lemon oil may help, but chances are after almost 50 years of use, a refinish is probably warranted.
Busted to Buff, Dashner Design and a few others are excellent YouTube sources. If any of the joints are loose, they will need re-gluing.
The seats are probably foam over plywood or hardboard. I have a "go to" solution for seating foam I've used for years. I use 1/2" carpet padding, readily available at DIY hardware stores.The foam is much more resilient than a lot of seating foam - "resilience" is how well the foam recovers once it is squished/compressed. Since it's only 1/2" thick , I simply cut and use as many layers as I need. I just stack them on the seat, I've never bothered gluing them together. I started down this path because foam is a 20 mile drive to downtown (one way) and carpet padding is a 3 mile drive to the DIY store. But ever since, I prefer the carpet padding for dining type chair seats.
As far as choosing an upholstery, the rub count is usually on the tag, but I've pulled $5/HD pieces out of clearance bins that have held up great. The only rule I can think of is this: the thinner the fabric, the tighter the weave needs to be. Basically, don't grab drapery fabric by mistake, though some of it will work too.