Quaker State is now owned by a conglomerate who buy their oil from other sources. They also own Slick/50 which should tell you something.
I've used Mobil 1 15W-50 in my 300E since the 1,200 mile mark. I've used it in Minnesota winters and Arizona summers.
Personally, I think any name brand oil will work just fine so long as it's not a 5W-30 or 10W-30. These so-called economy oils are pushed more for meeting CAFE averages than for long term reliability.