I have used Mobile 1...forever / prefer the 0-40w up here in Iowa. I recently switched to Royal Purple...which I had ignored all along since I thought of it as an Expensive Racing Oil. Yes, it is more expensive - up front.
I would change the Mobile 1 every 3000 miles.
I then (finally) read the Royal Purple web site from front to back - clicked everywhere & watched all videos, and found you can go 12,000 miles before the next change.
Pulled out the calculator...and it IS cheaper to use the RP vs. Mobile 1 under my conditions.
I stilll was skeptical about the 12K change, so I e-mailed RP, and the Rep (of course) stated "that 12,000 is a conservative mileage for our product"
I then switched my '95 Impala SS over to RP / got a confirmed 2.8 MPG gain over what I had been getting - SWEET!
When we purchased the Journey, I waited until 2000 (as recommended by RP) then changed over. Again I seen a 2.5 confirmed MPG gain!
I'm sold. I will be switching the tranny fluid over to RP soon to additionally lessen wear/tear, and improve my mileage (the RP website has video clips about improved performance & gas mileage by changing all fluids over to RP)
Hope this helps!