Here are two UOA on my wife's 2017 Subaru Crosstrek. She drives maybe 75% highway.
I felt I was absolutely wasting serviceable oil doing 6K mile changes, so I extended that ~ 50%
The older UOA is with Valvoline Advanced 0W20.
Traded car at almost 100K and the engine was running better, smoother than new and used no oil.
She has a long highway commute daily so an easy fit for and extended OCI. - Ken
View attachment 107766
hwy miles are kind on motor oil. This particular engine probably runs clean.
For EPA fuel economy, they test 45% hwy - 55% city. They think that is the more common split for average driver. So lets calculate the "highway equivalent". Lets use 2x as well as 1.5x for how much 'harder' a city mile is than a hwy.
10,000 oci combined = 4500 hwy + 5500 city
hwy equiv (assuming 2x) = 4500 + 2 x 5500 city = 15,500
hwy equiv (assuming 1.5x) = 4500 + 1.5 x 5500 city = 12,750
6,000 oci (subuaru rec oci) = 2700 hwy + 3300 city
hwy equiv (assuming 2x) = 2700 + 2 x 3300 city = 9,300
hwy equiv (assuming 1.5x) = 2700 + 1.5 x 3300 city = 7,650
your subaru, assuming 9K mil oci with 75% hwy:
hwy equiv (assuming 2x) = 6750+ 2 x 2250 city.............= 11,250
hwy equiv (assuming 1.5x) = 6750 + 1.5 x 2250 city........= 10,125
Now lets see oil % usage:
. ........................................................vs 10K OCI......vs.....6k OCI
Your subaru 2.0x city: ...............73%............................121%
Your subaru 1.5x city:................79%.............................132%