Just looking @ 2 cans of 10W30 from back in the late 70s/early 80s that have been kicking around our garage for like forever.
The Gulf (disappeared after purchased by Petro Canada) can has an API rating of SF/CC. The other branded William Penn/BP (another long gone brand here) has an SE rating. Obviously those oils were from the era of 3000 mile/5000 km OICs.
Then as someone surmised, once synthetics became more common, the OEMs found the 5W30 was suitable.
As I said earlier, there was no mention of whether to use conventional or synthetic. I figured if I was going to follow the OLS, I'd best use synthetic. Flame away, but w/ the mileage I was putting on my '95 K1500, it was nice to not have to spend so much time & money changing oil...