I’ll look this morning I’m guessing early to mid 1990s.
On a side note, the house I grew up in from second grade through college had an old drum of oil sitting under a tree when we moved in circa 1990. My dad was able to tilt the drum and get oil out, and promptly stated that was fine for the riding mower! It didn’t blow up so I guess he was right!
Looking back I would’ve loved to run a used oil analysis on that oil and have one more conversation with dad, even it is just so he could say I told you so.