Tricky one this to give a definitive answer because no one has been around for 30 or so years to have a look (inside) to see how their chassis is doing after treatment.
We need to travel into the future to have a look
That said, I'm of the school that says anything squirted or sprayed inside the chassis is better than nothing (short of battery acid).
I think the base line would be using old engine oil, maybe mixed with some diesel.
And you work up from there.
Cost very much comes into the equation.
Oil and diesel are cheap.
Some of the more exotic was treatments are quite expensive.
Another consideration is what are you going to use the vehicle for
If it's for pay and play in the mud and/or you regularly go off-road/farming work then you'll need more protection than most.
After a heavy off-road session you need to hose down/pressure wash out the crud from the chassis.
Don't let any rust traps linger.
Then re-treat the insides. This becomes very expensive if you are using the "best" wax.
I guess most owners don't want continual chassis maintenance and just want to spray something inside it and then forget it