High Mileage AC Recharge
Living in Florida I consider AC to be as essential as a running engine. I've been using this in my fleet of aged vehicles, and I have to say it's worth the extra cost. Definitely better than the $10 can of 134a from Wally World. It's reduced the compressor noise in every vehicle I've used it in.
PB Blaster
This is a Garage Essential that I've been using for 20+ years. Nothing is more frustrating that broken and stuck bolts. While it may not save the day every time, more often that not some good Penetrating Fluid will prevent a lot of headaches.
Sea Foam Trans Tune
Most people have heard of Sea Foam, and it's a great product that I definitely use. But most people haven't heard of their Trans Tune product. I've used this in both Transmissions and Power Steering systems with good results. It's not going to perform miracles, but it may give you old systems some new life.
Cooling System Cleaner
Another thing you should be looking in to the moment you get a new, old car is the cooling system. Even well cared for Rust, Scale, and Crud can build up over time. Whether you're having overheating issues, or just doing basic maintenance, a good cooling system flush is a great idea.
Surface Shield!!!
I know there's some PB Blaster up above, this is another fantastic, but different product from them. All the other videos are correct. This stuff is far and away the best undercoating, and rust prevention you can get. I started out converting and painting my rusty frame. Half way through I started using this and don't foresee any reason to strip and paint the rest. I'll just keep applying Surface Shield.