Almost all engines will use some at sustained 70 mph speeds for 500 miles or so in 90 degree + weather. All engines will use oil over a 7500 mile OCI, maybe not noticeable in 3-4000 but in 7500 I will bet the oil level does change and it should, just normal.
... cruising at
legal speed for how-many-hours should not consume any noticeable oil during regular oil change interval. unless you're in a S2000 cruising @ 4000rpm lol
Now if you're flooring the exploding dinosaur out of it going up the mountains like I sometimes do, for a couple of hours nonstop, it does burn oil.
That is why some people (like me) change from OEM specified W20 to W30. or even 40. zero consumption. (well, i filled it to F, it was still at F at the next change)
Sometimes, the manual's just the manufacturer opinion
(I mean they didn't build the car for racing uphill, so they said it's ok to use 20weight)
Let's word that a little differently, OK ??
It doesn't necessarily "wear out" but it does lose some of it's viscosity properties and become.........more like a 5W20.
shear is the word you were looking for
Also Royal Purple.....but I think you need to re-check Mobil 1.
Pretty sure they "cheaped out" after they lost the court case.
Not sure about their relatively new "Extended Performance" variety. Don't see how they could recommend a 15,000 change interval if it is NOT a "true" synthetic.
RP is not a real synthetic, it's group 3 VHVI just like walmart mobil 1. There's been this image around how "pure" RP's base oil is showing the inside of a big tank and shining light through, any gas station $2/qt base oil looks like that too. Don't be fooled.
All amsoil engine oil except signature (or those top-of-the-line) have changed to group 3 blend. like mobil 1
They should make a law to distinguish between "fully synthetically treated"(RP and most amsoil) and "fully synthesized"(amsoil signature & Redline).