They are when comparing across generations because of the rule differences, for starters. It was literally a different game. That is why pace adjusted stats and other advanced analytics mean squat across generations. I actually think you should agree with this based on what you wrote.
In regards to Oscar, I can't claim to have ever seen him play. So I'm not arguing where he stands in the top ten. My point is, you can't really unless you were one of the few to have seen him multiple times over multiple years still alive. The only thing you can ask is "Was he the best during his prime?" If he was, then there is an argument to be made for him as the GOAT. I wouldn't make it, but someone could, I'm sure.