Dropping MPG

<div class='quotetop'>QUOTE(donee @ Oct 6 2007, 08:13 AM) [snapback]522132[/snapback]</div>

But you can overfill with any type of oil, regardless of viscosity, and have the same fault.

Oil viscosity has its most significant effect on the engine at start-up - once running, the oil viscocity range you are talking about is insignificant compared to all other losses such as mechanical, friction and heat.

There are oils that last longer than others (i.e. degrades slower), but if you stick to the recommended car service intervals, this is not an issue as most engine manufacturers base their designs on the use of common, non-specialist oil.

<div class='quotetop'>QUOTE(donee @ Oct 6 2007, 08:13 AM) [snapback]522132[/snapback]</div>

No disagreement here with oil overfill effects, but overfill is a separate issue to oil viscosity grade, and the latter isn't a significant contributor to mpg effects.

 

Hi Jimbo,

The Prius engine only maintains temperature when on a long highway drive. During slow and go, or short trips, it can cool a bunch as the engine is not always burning gas, and in some cases not even turning over. People with Canview systems can see the cooling during a long glide. So, the viscosity of the cooler oil will be thicker, and effect mileage.

 

<div class='quotetop'>QUOTE(donee @ Oct 6 2007, 04:48 PM) [snapback]522220[/snapback]</div>

Well, that's why modern engines use multi-grade oils, so that the oil has a wider operating temperature range where the rate of change in viscosity is less. When using the car, even on short, slow trips, the temperature of the engine will still be relatively high compared to an engine that's been left parked overnight in winter time - and the recommended oil grade used in the Prius has a winter rating, which means it is able to flow throughout the engine from a cold winter start. Therefore, the variation in engine temperature on slow short trips will never be as severe as the variation in engine temperature between cold start up and normal operation - and the oil is able to function normally in the latter, so functioning normally in the former is no problem.

Also remember that going to a lower viscosity grade oil can result in less wear protection for the engine at high operating temperatures, so it's not a good idea to go off the manufacturer spec.

I think oil overfill is more of a culprit than oil grade, as well as fuel system contamination build up, in decreasing mpg.

 

<div class='quotetop'>QUOTE(JimboJones @ Oct 6 2007, 06:42 PM) [snapback]522240[/snapback]</div>

Manufacturer spec is 5W-30. People have had 10W-40 put in and that effects the mileage. 10 is twice that of 5. What does W mean 10 F? A 50F (the OP is in western Michigan, not southern Az) startup is allot closer to 10F than 180 F typical car running temp. Also remember the battery energy is used to turn over the engine at 1000 RPM continuously when in electric drive above 40 mph. And that energy ultimately comes from the gasoline burned in the engine. I think the drag of the oil would could have a large effect on the engine turning power.

Except for the Prius, Toyota has changed all its cars over to a lower viscosity, actually (0W-20?). The Prius was maintained at 5W-30.

 

<div class='quotetop'>QUOTE(donee @ Oct 6 2007, 06:36 PM) [snapback]522258[/snapback]</div>

The "W " rating signifies winter, and the lower the number before it, the lower the viscosity of the oil at "cold" temperatures (the W rating is usually viscosity at 40 C). The second number is the viscosity of the oil at 100 C, which is up around the normal operating temperature for the oil. Hence, 5W-30 oil will be "thinner" at startup temperatures than 10W-30 oil. However, once oil starts heating up towards operating temperatures (but well before it), the viscosity difference between 5W-30 and 10W-30 becomes zero.

Comparing 5W-30 to 10W-40, one can work out that the latter will be "thicker" at start-up, and also at operating temperature. One of the results of this will be higher oil pressure; another will be thicker coatings of oil for the metal components.

From the SAE Viscosity Chart (High Temp) at
http://www.upmpg.com/tech_articles/motoroil_viscosity/

going from 30 to 40 SAE Viscosity means there can be a zero to 7 seconds difference in viscometer oil flow between the two oil grades at 100 C. This may translate to no difference in the first (best) case, and a noticeable hit on mpg in the (second) worst case. So it is possible that people are experiencing mpg hits with the thicker oil, but it depends on the specific properties of the oil brand they use (best case or worst case) and the temperature of the oil during operation.

<div class='quotetop'>QUOTE(donee @ Oct 6 2007, 06:36 PM) [snapback]522258[/snapback]</div>

That's most likely due to better engine temperature management for the other Toyotas, i.e. cooler running engines, hence lower high temp viscosity oil (remember that lower viscosity means less protection at higher temperatures). It also seems to indicate that the other Toyota engines require thinner oils at startup, or Toyota thinks the world is cooling down, not heating up!