Amervo,
As Gary mentions, 6 quarts is what the '68 Owner's Manual and Pontiac Service Manual states and what I have used in mine for 36 years. That is the correct capacity for the V8 with standard PF24 filter change.

I just checked mine. With a 6 quart change about 20 miles ago, it shows a little above the full mark - probably a 1/4 - 1/3 quart or so above full. I estimate that with only 5 quarts it would be 2/3 to 3/4 quart low.

I have the original oil pan (engine never out of the car), original dip stick and correct filter.
Jim