MPG and oil viscosity

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Apr 19, 2004
New York
We know that 'thicker' oils are supposed to hurt gas mileage. Is the thickness when hot or cold more important in this regard. If a car is spec'd for 5w30 would mileage be hurt more running 5w40 or 10w30 (summertime of course)? My guess would be 10w30 would give better mileage but I wanted to see what others thought.
The one with the highest viscosity at startup will reduce the mileage the most. The 10w30 is probably thinner at 80-100°f than the 5w40.
All else being equal, the 10W-30 should give you better economy. The assumption is you spend more time at operating temp than not.
It all depends on how worn your engine is. Obviously, if you go and pour 40W into a brand new tight engine, you're going to lose fuel economy. If your engine has 85K miles on it and is well broken in, you might actually benefit from a thicker oil. Thicker oils will fill up the increased clearances from loosened up engines making them spin easier.
Than a 5w50 oil like Castrol Syntec seems like a great oil for older engines in moderate climates. I hope so since I am using it for the first time (yesterday) here in central KY. My BMW has 124,000 miles on it.
It depends on how long your driving events are. How many warm up cycles per mile (or miles per warm up cycle) will have the biggest impact on fuel economy based on visc. At full engine warm up (about 13-17 miles/20-25 minute from oil temp for non-exchanged engines) the visc is not all that much different compared to the starting visc difference. Anywhere up the line the 30 weight will take less energy ..but it typically doesn't add up to much on the upper end. That is, if you dyno'd for fuel economy the difference between the two at "steady state" would be barely detectable ..but would be measureable on the warm up cycle where all kinds of other sapping economy stuff is occuring too. That is, even though the 5w-40 has a 100C visc of 15cst (+/-) and a 10w-30 has a 100C visc of 10.x cst ..representing a 150% multiplier in visc ..that difference may only represent .01% of the total losses in operation at that temp. While the 70-90F visc difference, although about the same % wise, is substantially higher (a difference of 40 Cst as opposed to 4.8Cst - or 10X the difference @ 100C) The 40C temp should work for your "summer" scenario. The colder temps would exacerbate this difference and therefore pile on more losses [I dont know] Delvac 1 cSt @ 40ºC 102 cSt @ 100ºC 14.8 M1 10w-30 cSt @ 40º C 62 cSt @ 100º C 10.0 Just my opinion [Smile]
Thanks for the replys guys. GA: Yours in particular was very informative.
Why not just stick to 5w30? Faster warmup and the same fuel economy at operating temperature?
Originally posted by mechtech: The best mileage should be: 1. 5-30 2. 10-30 3. 5-40
Yip, you got it.
[QUOTE]Originally posted by BlazerLT: [QB] Why not just stick to 5w30? Because I have several quarts of each that I need to get rid of.
If there's ever a good time to use up the 10w-30, now would be that time. It's starting to get warm, so the 10w-30 won't put as much of a drag on the engine at startup than it would in the colder weather. Also, common knowledge is that 10w-30 is more stable that 5w-30 in hot weather (although 5w-30 has certainly come a long way). I agree with the others who say that it depends on your driving, though. If you're more prone towards longer trips, you'll probably see no difference. If short trips, you might see some drop in MPG compared to short trips. I can't ever see 10w-30 causing harm in an engine spec'd for 5w-30 except in very cold weather.
For what it is worth, when speaking of fuel economy, tire pressure plays a much bigger role than oil viscosity. Rolling resistance is much greater than the frictional or pumping difference between a 30w and a 40w. Easily a measurible difference between 30 psi and 35 psi. By the way, your tire pressure goes up about 1 psi for every 10 degree F increase. (same way on the bottom side)
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