It's the engine, not the oil. Also that a LOT of oil for a 1.5L engine.
Does your small 3litre V6 have a 8 qt sump to be comparable? Does you v8 have a 12-15 qt sump? I think not.
M1 = nothing special - ask my Subarus - Nice browm Varnish and deposits on the cam cover and VVT box.
Saw None with Valvoline 0w20, and it cleaned up the M1 and Shell products varnish mess in just one OCI.
Now If you want a synthetic, The 0w20 EP is purported to be XOM only synthetic oil ( majority group iv) per the SDS. I don't think you can trust the SDS, though. And when PAO hits an OCI wall it hits HARD.
If I had a stick car that used a 20 grade I d try it for you and report my qualitative findings.
I don't plan on buying any cars that use 20 unless its a ford 6 or 8.