[off topic] fuel economy and synthetic engine oils

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Apr 28, 2003
Northern Virginia
For anyone interested, I have been running Shell Rotella T synthetic 5w40 for a while (WalMart, $13/gal) and recently switched to M1 10w30 (WalMart, $17/5 qts). Fuel economy consistently was 5% lower w/ the M1 10w30. Fuel was the same (chevron 91, mtbe additive), driving routes were the same (commuting, avg speed of 35-55mph, 22 mi each way, most flat, temps around 80-85/50-60F in So. AZ).
Vehicle is a '86 4runner, 22re 4-cyl, 153K mi, new valve seals/cam/rockers/exhaust valves/HG/header+cat+catback 25K mi ago, original lower block. No cruiser content, other than how to possibly improve fuel economy based on the choice of motor oil. :) Also, oil burnoff in my '01 honda prelude SH (1 qt per 3K mi on dino 5w30) was eliminated after switching to M1 Delvac 5w40. That's obviously a thin oil issue. Damn CAFE! I saw that the recommended oil in Japanese motor mags is M1 5w40 or other brand 10w30's and 10w40's, not 5w30's (guess they think 5w30 is too thin?).

Here are some responses....
I just started useing Rotella T 5w40 in my Wife's 2003 VW Golf 2.0 about 4 months ago. Initially, I used Mobil1 5w30 immediately after break-in. We decided to switch over to Rotella strictly based on lowering maintanence cost since I change it every 3-5k miles. (...also because 5w40 is the recommended weight per VWoA) We've noticed no change in MPGs with the switch, just a slightly fatter wallet.
It's not Rotella in particular, it's the thicker oil. I have seen the same thing when I switched from 10W-30 to 10W-40 in my SAAB and so have other people. Try any 10W-40 or 5W-40 oil and you should see same fuel economy numbers as with Rotella.

The theory I have is that the thicker oil reduces internal friction more than energy wasted due to thicker oil. This seems to be the norm for engines with 70k-80k miles or more.
40 wt. might also create a better ring seal than thinner oils in higher mileage engines.
If that's an '86 4Runner, then I suspect that it has a few miles on it. If that's so, then you are probably getting some blow-by with the lightweight M1, whereas the heavier 5W40 is giving you better compression and therefore better fuel economy.
I just picked up Rotella T synthetic 5w-40 for my next oil change, it's significantly cheaper than the M1 10w-30 I've been using, and I hope it helps with consumption. Fingers crossed!
When did you switch? I notice a difference in mileage when gas stations switch from summer blend to winter blend.
What's the difference between the 5 or 10 before the W and the 30 or 40 after?
Trying to pinpoint a 5% MPG difference with accuracy is highly improbable but not impossible.
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Yeah I am with darkness. I would bet any difference seen was due to more ethanol in summer gas mix.
We definently notice the summer/winter blend thing as well. As far as oil, find one your vehicle and wallet likes and go with it. What I've noticed over way too many years is that certain vehicles like certain oils as well as fuels better. It's just like women prefer certain wines over others. Give them what they like and life becomes easier.
What's the difference between the 5 or 10 before the W and the 30 or 40 after?

Cold and Hot weight. If you live in cold climates you might go with a lower cold number... On my Jeep in VT I run 10W-40 summer and 5 or 0W in the winter (just depends on what's sitting around). Just do two oil changes a year, sometimes no filter as it doesn't get that many miles on it.

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