Why better gas milage from 5W-40 vs 15W-40

[wwillson]

All,

I have run Mobil Delvac 1300 15W-40 in my 5.3L Suburban (36,000 mi total) for about 10,000 miles. I put Shell Rotella Syn 5W-40 in about 2,000 miles ago and find that I get better gas milage than with the 15W-40. With the 15W-40 I normally got about 15.5-16 MPG, with the 5W-40 I'm getting about 16.5-17.5 MPG. I have also noticed that with the 15W-40 it would hold about 65 psi oil pressure. With the 5W-40 it holds about 50-55 psi oil pressure.

My questions are: Why am I getting better gas milage with the 5W-40 vs the 15W-40? Why does it hold more oil pressure with the 15W-40 when the 5W-40 is has the same high temp viscosity? Is the 5W-40 just simply easier to pump for some reason?

Wayne

 

[Propflux01]

The 15w-40 has a thicker base oil to start with.

 

[TooSlick]

Most recent Amsoil data (3/7/03), I saw on the Rotella T showed a high temp/high shear viscosity of only 3.93 Cp, vs 4.59 Cp for the Mobil Delvac 1, 15w-40 and 4.54 Cp for Delo 400, 15w-40. The Rotella is thinner than the Delvac 1300 even under steady state conditions ...it's also thinner during the warmup phase.

It's simply a matter of fluid mechanics and viscous losses inside the engine ....

TS

 

[wwillson]

quote:

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Originally posted by TooSlick:
Most recent Amsoil data (3/7/03), I saw on the Rotella T showed a high temp/high shear viscosity of only 3.93 Cp, vs 4.59 Cp for the Mobil Delvac 1, 15w-40 and 4.54 Cp for Delo 400, 15w-40. The Rotella is thinner than the Delvac 1300 even under steady state conditions ...it's also thinner during the warmup phase.

It's simply a matter of fluid mechanics and viscous losses inside the engine ....

TS

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TooSlick,

Thanks for your reply.

Does the lower viscosity of the Rotella Syn mean that it's not as good as the Delvac or Delo during high heat/shear conditions?

 

[TooSlick]

No - they simply formulate it thinner to begin with ...

A thin, 5w-40 is better for gas engines that spec 5w-30, than a thick, 15w-40 ....


TS

 

[csandste]

I switch from 10w30 to 5w30 during colder weather and can certainly tell that there's an increase in gas mileage. Base oil weight does matter.

 

[Sin City]

To put it very simply Rotella T Synthetic 5W-40 is much thinner when cold and a little thinner when hot compared to just about any 15W-40 "diesel" oil. The difference in fuel economy that you mention is very significant in my opinion. I imagine in your situation it is cheaper to run the Rotella T 5W-40 than the conventional 15W-40 oils when you consider the added fuel economy of the Synthetic oil.