Can someone explain the differences between PP and PU? Since PU is becoming harder to find I am considering switching to either PP or Mobil1 EP in a 30wt grade. Not sure if it will be 0W30, 5W30 or 10W30. Would like to understand the differences before I do so though. It does look like I can order PU online and have it shipped to the local Walmart if it is not stocked.