Ive run Castrol Magnatec in 5W30 and 5W20 flavours, I got some right at the rollout years ago when it was labeled " semi-synthetic".
This oil does have an ability to calm and quiet various engine noises, but I too find it a bit "gooey" and ultimately power robbing.
Having tried many oil on a problematic, noisy engine - the big Nissan 4 banger -QR25DE, The early release Magnetec in semi--syn was very very good. I haven't like the later formulations as much, though - Not as Velvety; and neither are super "clean" running.
I will say 5W20 Magnatec works well as a "companion oil" with other oils in "calming" certain engines. Add just a pint; Think of it like an additive. A whole sump of the stuff may be too much of a good thing.
But Of all the tens of oil brands I've tried over the last decade on Subaru and Nissan and Toyota, Nothing has come close to the Valvoline Advanced ( NOT modern engine** )especially in the 5W30 flavour.
It runs like an "ultra premium" boutique synthetic, providing better power, lugging torque and high fuel mileage together.
So it's not PP or Maggie it's Valvoline - at least in in GF5
BTW the PP has never worked well enough in anything I've tried it in. When I tried it In the Subaru, It drank it like a Saturday night Frat party.
- Ken
**I have not tried Modern Engine