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

wwillson

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Naperville, IL
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
 
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5,785
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Dixie
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

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Naperville, IL
quote:
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
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?
 
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5,785
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Dixie
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
 
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509
Location
Las Vegas, NV
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.
 
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