What should I use on wood of very old Goat Cart to preserve it?

exterior marine varnish

Wood needs oil. I use wood cleaner that is oil based on my furniture. Work it in with a soft cloth. You might have to give it time to settle/dry and then add another coating.

Tung oil works nicely, but rub it on in thin layers, don't goop it.

To preserve wood - olive oil is very good.

I would look on this DIY site for information. Whenever I have a question about taking care of my furniture or my flowers this is where I go.

My Mother always used Scott's Liquid Gold. I now use it on all my wood furniture too. Keeps them looking like new.

Have you tried painting it with polyurethane? That stuff can seal just about anything! Just go to your local hardware store and ask their advice on which one to use.