Thick Oils and MPG (relates to VW/Audi 2.0L)

[The Critic]

I have a question for the owners of the VW/Audi 2.0L Engine on this board.

If Mobil 1 Truck & SUV 5w-40, a thick and shear stable 40wt is used in this application, how much of a fuel economy loss is expected?

In this application, the car is used for a 10-15 mile daily commute at highway speeds, twice a day. From my understanding (correct me if I’m incorrect), if a commute is within 10-15 miles, the oil spends much of the time “warming up,” and may not reach the normal operating temperature during the commute.

In this case, wouldn’t there be a significant loss in fuel economy using Mobil 1 Truck & SUV 5w-40 since it is rated at 102cst @40C, compared to most Mobil 1 5w30 for example, which is 64.8cst @40C. What I mean by this is that since this oil is thicker during the “warm-up phase,” wouldn’t it cause a significant decrease in fuel economy?

In general, wouldn’t an owner see the least negative impact on fuel economy if thicker oil was used on longer commutes, such as 50+ miles? I was thinking that there is a lesser viscosity difference once the oil is warmed up, since 14.8 cst for Mobil 1 Truck & SUV 5w-40 vs. 11.3 for Mobil 1 5w30 is a lesser difference compared to 102 cst vs. 64.8 cst at 40C for the Mobil 1 Truck & SUV and Mobil 1 5w30, respectively.

Thanks,
Michael

 

[Spitty]

For what it is worth, a close friend of mine has a 2.0L Golf and her last oil fill was Dino 15w40, she keeps an accurate record of gas mileage and has been paying extra attention since the last 'Accidental' oil change. So far there has been no significant difference in fuel consumption between the usual Dino 5w30 and the 15w40.
Her usage consists of 2 mile downtown shopping trips plus twice weekly 1 hour commutes to the city.

 

[huffmuds9320]

Well I haven't used Mobil 1 5W-40 Truck and SUV in my VW Jetta 2.0L before. However I have used 5W-40 synthetic, 5w30 synthetic, and 5w30 conventional. And I haven't noticed any difference in gas mileage between the oils except for oil consumption. It consumed the most oil with Mobil 1 5w30 and least with 5w30 conventional. So Mobil 1 5w30 is probably not the best choice for your 2.0L. But I think the 2.0L isn't really sensitve to oil grades or kind. Since you're in nice warm California, cold starts shouldn't be any problem. Most VW manuals say 5W-40 is preferred. So I think there wouldn't be any huge noticeable gas mileage difference.

I get about 21-23mpg city and 25-28 freeway/city mix.

[ May 17, 2005, 02:11 AM: Message edited by: lpcmidst128 ]

 

[Steve S]

Don't know why anyone uses a o5w/ syn.oil.

 

[mechtech]

Steve - The lower the better [for the first 'W' number].
You want as much flow as possible, whether it is winter or summer, on start up.
The oil is ten times thicker on start up.

 

[y_p_w]

quote:

---

Originally posted by mechtech:
Steve - The lower the better [for the first 'W' number].
You want as much flow as possible, whether it is winter or summer, on start up.
The oil is ten times thicker on start up.

---

Flow at warm weather ambient temps may not correlate well to the "W" number. Strangely enough, the latest spec sheet for Mobil 1 has their 10w30 thinner than their 5w30 at 40 deg C (105 deg F). Mobil 1 0W-40 is considerably thicker at room temp than a typical 5w30 or 10w30. That T&S 5W-40 is going to be thicker than a 10w30 for warm weather starts. The "W" sample point is some ridiculously cold temperature.

http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil_1_10w-30.asp
http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil_1_Truck_SUV_5W-40.asp

In summer temps, a typical SAE 30 oil will be thinner than a 5w30 at winter temps.

However - all this is fairly moot because once it gets to summer temps, almost any off the shelf motor oil should provide easy startups at ambient temps.

 

[moribundman]

Does the 2.0l VW engine have low-tension rings?

 

[TooSlick]

Fuel efficiency of reciprocating,internal combustion engines is closely related to the high temp, high shear viscosity of the lubricating oil; tested @ 150C.

A good first order approximation is that fuel efficiency is INVERSELY proportional to the cube root of the HT/HS viscosity (in Cp) @ 150C....

 

[Dr. T]

Don't worry about the piddly loss in fuel economy which only happens initially. The cleanliness factor provided by the 5-40 in addition to better piston sealing will more than offset this.

 

[TooSlick]

The drag on a boat hull goes up by the cube of it's velocity. Now imagine a boat hull being pushed through a thicker fluid - say a silt thickened river. The drag in that case would be proportional to the cube of the velocity, times some constant. This constant would reflect the different in the relative viscosities of water, to water thickened with silt.

Now lets look at oils of different viscosities being pumped through restricting orifices throughout the engine - but lets look at it in the reversed "reference frame"....Take a large tank full of oil and heat it up to operating temp. Then push a bearing orifice throught this medium. Again the drag would be proportional to the velocity cubed, times some constant which reflects the viscosity of the fluid at that temp. So, in order to determine the drag for a fixed velocity, you would simply take the cube root of both sides of this equation. Therefore, when pumping oils of different viscosities at a fixed rate or velocity; the pumping losses should be proportional to the cube root of the viscosity. In this case I think it makes sense to use the HT/HS viscosity and not the kinematic viscosity @ 100C.

This idea came to me while I was out for a run the other day, so it may be totally off base. However it feels intuitively correct to me ....

Tooslick