I recently purchased a 67 Firebird that someone swapped a 428 into. I just changed the oil, but I am having a hard time figuring out how much oil to put back in. The dipstick appears to be aftermarket and I don't trust it. The oil pan is the original style with the drain plug off to the side. After putting in only 4.5 quarts, the dipstick shows full. Considering I changed the filter as well I was thinking it SHOULD take 5 quarts plus a bit more for the filter change. I went ahead and added 5 full quarts and now it's showing passed the full line on the dip stick.

Is it possible I didn't get all of the old oil out? The oil wasn't very warm when I changed it due to the proximity of the oil filter to the headers. I had to let it cool off some.

Should I add another half a quart to be safe? There is so much conflicting information about the actual quantity needed for this engine.