Since all I had laying around to put in my '76 Torino with the 351M was 4qt of 5W30 and 1qt of 10w40, and it clicks and ticks like crazy, I figured it needs to be thicker. 2qt of this 50W stuff and it's mostly quiet except for a single clack from what I believe to be a stuck valve. I'll be changing the oil again in about 800 miles, want to get something in there that isn't a mishmash, and 5qt bottles are perfect for that. Too bad HDEO isn't sold in 5qt, only gallon. But the truckers need gallons for their rigs so I understand.