MPG 30 vs 40 wt

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Jun 1, 2002
Long Island NY
I know there was some previous discussion here on the difference 0-30 vs 10-30 makes in MPG. Has anyone seen any noticable difference in MPG when switching from x-30 to x-40 wt oils? I like M1 0-40 but wonder if there actually would be a noticable drop in MPG compared to 0-30.
VaderSS recently went from 5w30 M1 to 0w40 M1 and reports his MPG is about the same. Oz also reported his MPG is the same or better when he went from 5w30 Tech 2000 synthetic to 5w40 Delvac 1.
Mark, As Patman said, I did not notice a big difference in MPG. Here are my results after using both Tech2000 10W30 Synthetic (WalMart brand) and Mobil Delvac 1 5W40. Tech2000 10W30: Average 7.28L/100km. Best MPG is 5.98L/100km. Worst MPG is 9.02L/100km. The oil was subjected to a lot of highway driving (Montreal to NY City; Montreal to Ottawa). Oil analysis of this oil can be found in the Used Oil Analysis Forum. Mobil Delvac 1 5W40: Average 7.48L/100km. Best MPG is 5.94L/100km. Worst MPG is 8.83L/100km. The oil was subjected to a better mixture of highway and city driving (roughly 50/50). This is using CarCare2000 software. Link can be found in the Links forum. Regards, Oz
It's only significant to the bean counters at the auto mfr's to avoid CAFE penalties when added up over millions of cars sold. We are talking fractions of an mpg.
Thanks Guys, I think I will stay with the Mobil1 0-40 that I have been using recently. Chaulk up the poor MPG to August A/C use and more than usual around-town driving.
Mark, I've run the two extremes in my car, a 0w-30 synthetic and a 20w-50 synthetic. A 10 tank average with the trip computer showed about a 4% gain with the 0w-30 (24 mpg, vs 25 mpg). I'd say you lose about 1% for every SAE grade you go up in viscosityh, so going from a 10w-30 to a 10w-40 costs you about 1.0%. Going from a 10w-30 to a 15w-40 would probably result in a difference of 1.5%. There is an additional fuel efficiency benefit to the low viscosity oil during the warmup period. If you do lots of short trip driving you will see more of a mileage difference between different SAE grades than if you do long highway trips.
I've run 10w40 instead of 10w30 in my car for a couple of changes. It always gets over 30 mpg, so any small percentage would show up more than with something that got lesser mileage, & I check mileage on every tank. I never saw any difference in mileage.
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