Speculation: 2008 requirement for Mobil 1 and high-octane fuel is for MPG

[nosro]

Consider the following:
1) For 2008, Cadillac makes Mobil 1 a requirement
2) For 2008, Cadillac makes premium fuel a requirement
3) 2007 and 2008 engines have no change in power rating or any announced change of any kind.
4) New EPA mileage testing procedure takes effect for 2008 model year vehicles (which tends to generate lower mileage numbers for the same exact vehicle)

I would speculate that the premium fuel and Mobil 1 are part of a coordinated effort to reduce the impact of the new EPA test procedure on the quoted mileage rather than for any design change or newly identified problem. (In an EPA test, anything stated in the owner's manual as a requirement is used during test. So if the owner's manual says premium is required, then premium is used during EPA testing.) The implication is that you can use regular oil and regular fuel in your 2008, just as for 2007 vehicles.

For the record:
2007: 17/25MPG (city/highway) (old test procedure)
2008: 15/23MPH (city/highway) (new test procedure)

[nosro]

As another note, the Northstar was never a "high-stress" engine. The Corvette engine, for example, stresses its oil because of the removal of an oil cooler to save on parasitic engine losses. Conventional oil could be cooked by the lack of an oil cooler, but synthetic oil would survive just fine. Thus Corvette engines require Mobil 1 or other equivalent synthetic.

[Superjim]

My 2006 says PREMIUM RECOMMENDED but it does not say REQUIRED.

The couple of times that I accidently put regular in it... it impacted both my mileage and performance.

Texas Jim

[mikeythegreek]

i just got a 2007 dts, Im still waiting on the manual but the service guy there told me 87 octane is fine and non synthetic is fine, im more worried about not voiding the warranty. What he did mention is a bulletin on gas companies that do not use a good level of detergents - gm, bmw, mazda, and vw went in on the testing - check it out - http://www.toptiergas.com