Olds V8s of that vintage usually hold closer to 6 quarts full, so don't panic too much.
I'd say the drain-back holes are not likley to be plugged if it's a 'decent condition' or low miles engine. It would be sucking the oil through the valve guides and burning it, if that were the case. Anyway, a good look inside the rocker arm compartment should reveal any sludge problem.
Most likely, it was previous owner incompetence, or a sloppy quick lube 'mechanic' that caused an over full crankcase. I know there are still plenty of people out there who think it might be a good idea to add a quart of oil occasionaly, just because they have an old car, or that's what their grandpappy always used to say, etc. etc. etc.
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