As one or two recent threads on the subject have shown, no one knows for sure what PP's made from. They appear to keep changing their recipe.
I agree that the venerable 2.5 liter Subaru "Boxer", "H-Pattern", 4 cyl. engine, in which the cylinders are opposed instead of inline, is a noisy engine. A lot of people say that it has a noise that's all its own. I have one in my wife's 03 Forester. I've run Mobil 1 in it for 18,000 miles and the Mobil has not increased the noise.
