Let a Prius owner chime in
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A Prius's engine is computer controlled & frequently turning itself On/Off pending driving condition, speed etc. Therefore, cold starting performance is tested very extensively.
Consequently, a Prius owner gets many repeated opportunities listening to how his engine is behaving after an OC.
While my Prius is under factory warranty dealers performed OC & 5w30 conventional were used.
Best MPG recorded =
47 mpg
When I started using synthetics at 60K, Mobil 1 5w30 was my first choice, the best recorded mpg was
48.9 mpg, not bad IMO.
Tried German Castrol 0w30 just once & contrary to my expectations it did not perform as well as Mobil 1, best recorded mpg =
48.4, as others have pointed out, the GC 0w30 has a higher than average viscosity at warm temp & I find that absolutely correct.
Once reached operating temperature, every time the engine is restarting I can feel more drag from GC than Mobil 1 & there was more start-up vibration resulted from a heavier oil.
But does this mean GC is not as good as Mobil 1 ? Absolutely not, because Prius's engine is fairly low in output & HP comparing to your Z-06's LS6, and it max rpm is 4500. IMO the GC is over kill. It is obvious GC is made for high performance engines.
IMO your Z-06 could benefit from the GC's higher viscosity, which is almost 40 wt.
Not long ago I switched to Royal Purple 5w30 after scored a 12 qt deal on eBay for $87.95 shipped. I immediately noticed a difference, my cold starting vibration is non-existent & engine is quieter. Something in RP makes it more "slippery" than anything else I've tried.
Remember, a Prius owner gets to hear his engine restarting over & over.
I ran the car out of gas on-purpose just to see how far I could get per tank using RP. One day my car stalled at 586 miles & drove 2 mi on battery until I found a gas station, took 11.4 gallon to fill up, which equates to
51.4 MPG, the best I've seen.
Keep in mind that I drive normally by keeping up w/ traffic & don't practice any "hypermiling" techniques to slow down the traffic & make people angry.
While I haven't tested Amsoil, Redline, Lucas etc. Royal Purple has given me the highest MPG after having used German Castrol 0w30 & Mobil 1 5w30.
Royal Purple 5w30 = 51.4 mpg
Mobil 1 5w30 = 48.9 mpg
GC European Formula 0w30 = 48.4