I decided to clean the spare tire and the spare tire well (yes, I really did - don't lie, I know you have too). I was looking at the spare and it occurred to me that the rim looked original. That's not that unusual. But then I started looking that the tire and I think this might original as well.

What do you think?

Sure looks original. Seems like someone was looking for one of these RD-150's for an restored 240 here once.

It's original, and I believe I have the same.

And yes, I too have taken the wheel out just to clean it and the tire well

I was torn between keeping the original spare which I would not trust in a pinch, versus buying a new nearly-original sized replacement. Still driving around with the original, not sure how well I'd expect 40 year old rubber to hold up for even a single drive back home.

The old tire will do just fine. It is when they get outside and exposed to the weather they develop problems. I wouldn't worry about it at all. I am not worried about my inflatable spare in my 77 for the same reason. I AM worried about the can of air next to it however...........

Is there anything I should do to treat the rubber, to prevent dry rot or cracking or anything?

I need to get a can of air for mine, I imagine I'd go to use it in a pinch to find it low on pressure. Haven't aired it up in more than 5 or 6 years, and I figure it's a pointless strain on the tire to keep it charged up all the time, given its age.

It's not that I wouldn't trust a 40 year old spare, but I'd hate to use it. After all, once used it can't be new ever again. I think I might store it in the basement and put a bazooka subwoofer in its place.