Trying out conventional 10w-40 after running synthetic 10w-30 and am seeing a drop in mpg. Was aware of the hit when running 10w-60 in my 2000 M5 vs 5w-30, though I chose to continue to run the heavier stuff. Wasn't expecting it here.

Anyone know if switching back to synthetic, but sticking with 10w-40 will improve fuel efficiency?

And yes, I know 'fuel efficiency' is an oxymoron in DII land.

Thanks,
Dave

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04 Discovery SE7
78k Miles

 

What are your actual MPG numbers?
The oil has nothing to do with it, it is still a "10" weight oil.

 

Thanks for the reply, Spike. You know, I also usually run Shell, but had to fill up on Chevron, so my results could be attributed to a number of factors, as well as different driving patterns.

As for mileage, I get ~16mpg highway only (fully loaded up & down mountain runs...people, gear, etc), and 12.5-13.5 local (combination hwy and city).

I'll wait until I can fill up with Shell and see where I am mileage wise 20-22 gallons in.

Thanks,
Dave

 

Without actual MPG numbers with the 2 different motor oils...

 

A 10w40 will be thicker at all temps when compared to a 10w30 so sure you should have expected a drop in mpgs but not really that noticeable. Are you sure you are not seeing the difference due to summer/winter gasoline?

 

What you may not have taken into consideration is most states have recently switched over to their winter blend gas which will always cut a couple mile per gallon.
Oil weight wont make that sort of change.

 

Spike, you're thinking of the 10W part, which in fact is the same for both, but at operating temp the 10w40 is going to have a higher viscosity than the 10w30.

 

Thanks everyone, this is helpful. Do any of you switch to 10w-30 in the winter? Looking at the owner's manual protection/engine temperature levels, 86 F is definitely too low for summer, but fine for my winters.

Thanks,
Dave

 

Even though they are both "10w" the 10w40 is still going to be thicker cold.

 

I use 5w40 year round. Switching from 10w40 to 10w30 in the winter really will accomplish little even thought the 10w30 is a bit thinner at start up. Change down to a 5w40 or a 0w40 and you are getting somewhere.