1.5 quarts of usage with no evidence of oil on the ground would signify the oil is being consumed, not leaking. Thats a lot of oil to seep and not hit the ground
Do you plan to run your cars out to 700k? Remember, this is 500k miles they ran. The engines still ran (even though rings were stuck and obviously burning increased) which is pretty remarkable.
Also, we don’t know what that engine looked like inside at 300/400k. It may have been slight varnish...
Napa is fantastic oil, motor oil geek did some testing with it and it did really well. Also, Kirkland/supertech has proven time and time again to be able to hang with higher priced oils and you can get it at a bargain price
if it were me, I’d personally just return the HPL, if possible. That same money could buy a lot of Valvoline Restore and Protect which has tons of proof to clean things up in an engine.