From an Automotive Parts Interpreters point of view who sells oil to mechanics on a daily basis and has to look up the correct grade/rating:
Its not so much of a worry in an older vehicle as the older they are (we're talking 60's to early 80's) the average family hack wasnt performance orientated.
You wouldnt be putting a fully synthetic oil in one of these, nor a semi-synthetic. Minieral based is how the oil was in those days.
Once you start playing with twin-cam, fuel injected and turbo motors, they start producing a lot more power, therefore the right oil with the right ratings should be used.
Good examples:
Datsun/Toyota/Suzuki/Ford/Holden - Early 60's to Mid 80's - If not rebuilt, go by KM's the engine has travelled, engine condition (does it blow smoke?) etc
A lot of early European cars still require the right oil (BMW 2002) for example, so you can't go with the above.
Buting anything performance orientated (4AGE, 1JZ/2JZ, CA18(DET), SR20(DET)) and similar, including the Euro cars, you're going to want to put the right spec oil in the car.
A few of links for you to check out, if you're interested in looking up oils for specific cars:
Fuchs Netlube
Castrol Netlube
Valvoline Netlube
These will tell you the correct grade of oil to use in whatever vehicle you select.
Enjoy! :)