Should you use 6 year old oil in a 40 year old car? Yes
I would probably trust a old container of name brand oil, just as much if not more, than a generic cheap brand oil made today. Within reason of course. I would still want the old oil to be sealed and meet the API spec (or better) of what is required by the car in question. Given this oil is from 36 years into the future from when the car was made, it's probably better than anything used when the car was new.
I've heard of people running much older oil than this without any problems.