Over thinking engine oil - MPG related

[EchoThat]

I wonder what I could get from my 2002 automatic Echo if it were re-programmed. It was EPA rated at 29/37, but I get 40 in the suburbs now with 168k miles on it. Highway is a bit better at 43 or so. But back to the original topic of the oil... I've used a 5w30 SuperTech synthetic in it almost exclusively since 25k. Burning about a quart now per 1500 miles. Going to switch to a high mileage 5w30 on next oil change in a thousand miles or so and see what happens. I have never gone much over 7k between changes.

 

[Gokhan]

Oil viscosity is always a difficult compromise, even for automobile makers. One should first understand that MPG is almost entirely determined by the HTHS viscosity and not really by the SAE viscosity. The same is true for engine wear, which is determined more by HTHS viscosity than the SAE viscosity. (Friction modifiers and antiwear additives are also crucial for MPG and engine wear.) The change in fuel economy between HTHS viscosity 2.6 and 5.0 could be as much as 5% or more -- a significant impact on the wallet. While it's true that higher HTHS tends to protect more, gasoline engines operate at lower torque and higher RPM, which decreases the critical value of HTHS for oil-film breakdown (which happens at low RPM, high torque, low HTHS viscosity, or a combination). So, most modern gasoline engines can tolerate HTHS viscosities as low as 2.6. One should also keep in mind that HTHS viscosity shears as well and a starting 2.6 could shear to as low as 2.0 after as little as a few hundred miles. Also, few people intend to keep their cars more than a few hundred thousand miles, and the lower-wear benefits of higher HTHS oils may not be needed for anyone who intends to sell their car after 200,000 miles or so. All this said, I'm a high-HTHS guy, curently running 4.3.

 

[Mr_Incredible]

In my personal experience it doesn't make much difference going higher OR lower in oil viscosity. In my 98 LS1 I did UOAs after using the thinnest 30w (Hav DS), a couple differerent 30wts (GC, PYB), up to the older M1 5w-40 TSUV. All the UOAs were indistinguishable as far as wear materials. In my 89 CRX I tried GTX 5w-20, though it specs a 30w in most instances. The wear moved from 0-3ppm to 2-4ppm, on average. I don't see that as a huge enough difference to freak out and stop use. I really beat the snot out of it when using that oi, too. Perhaps it depends on the engine itself, and UOAs to see what it will take. In the CRX, now, I'm going a bit thicker to drop the oil use between oil changes. I got 40.7mpgs out of my last tank full. The big difference in mpgs is up to the driver's style. Any increase in mpgs by use of a thinner oil is will within background noise of driving styles and would be a moot point. IMHO.

 

[OVERKILL]

Those numbers are well within the "noise margin" for inexpensive UOA's.... So I don't think the UOA's really told you anything.

 

[Mr_Incredible]

Certainly the usual UOA isn't up to accuracy standards of, say, any nuclear research lab. But they'll do for $25. It's at least better than killing a chicken over the used oil and throwing the bones. But also, your point is pretty much my point. Switching to 20wt isn't an instant death sentence for your engine. The difference on mine was not significant. Some engines may be prima donnas, but that one certainly isn't. No true difference on an already-279k mile engine.

 

[OVERKILL]

Originally Posted By: Mr_Incredible

Certainly the usual UOA isn't up to accuracy standards of, say, any nuclear research lab. But they'll do for $25. It's at least better than killing a chicken over the used oil and throwing the bones. But also, your point is pretty much my point. Switching to 20wt isn't an instant death sentence for your engine. The difference on mine was not significant. Some engines may be prima donnas, but that one certainly isn't. No true difference on an already-279k mile engine.

What I was trying to say is that the minute variances in PPM between the two is not enough to draw any sort of conclusion from. Inexpensive UOA's are a great tool for monitoring contamination, oil life, and to check for things like coolant leaks, air intake tract leaks and the like. But they are not meant to contrast one oil brand to another or one oil weight to another. UOA's are not to be relied on to tell you how your engine is "wearing". They can show you if there is a problem (large spike in a particular wear metal like lead or copper for example can indicate a bearing issue) but due to the rather narrow range of the sample particle size, they are not a "wear" determination tool. Think of it this way: Oil A allows an engine to shed 5ppm of FE at 2-microns and 50ppm of FE at 8-microns over 5000 miles. Oil B allows the same engine to shed 9ppm of FE at 2-microns and 1ppm of FE at 8-microns over 5000 miles. In a UOA, the FE in sample A would show lower than sample B, because the UOA doesn't detect the larger 8-micron particles. So people would then argue that oil "A" is providing better protection and lower wear than oil "B" based on this. Yet in reality, oil "B" is protecting the engine better, since it is shedding fewer large iron particles. I probably sound like a broken record, but Doug Hillary wrote up a very nice article as to the accuracy and value of inexpensive UOA's. If you haven't read it (which from your posts, it sounds like this is the case) then I highly recommend it: http://www.bobistheoilguy.com/used-oil-analysis/

 

[Mr_Incredible]

Anyone that's been here any length of time knows about the tiny bits vs chunks debate. One can presume, I suppose, that if chunks aren't mentioned they've been dismissed as part of the assumption/equation. Most of us live in a world of generalizations measured in 1/8th inch, at best. I seldom have the money, need, or time to measure anything to the 100th or 10,000th. I do when I have to, but for my purposes a UAO for 25 bucks is pretty high zoot by itself.

 

[Artem]

A $25 UOA gives you a general idea of whats going on. Any UOA, regardless of price should be taken with a dash of salt. It's not exactly 100% scientific fact which can be replicated with accurate results. That's like saying a 5qt jug of synthetic oil that's on sale for $20 is not very good when comparing it to retail @ $40 for the same oil. (meaning, some labs charge more for the exact same UOA) I simply can't see myself spending much more then B/S charges for a UOA. I add TBN, TAN and particle count and all of the sudden it now becomes a $50 UOA.

 

[Gene K]

Originally Posted By: A_Harman

Originally Posted By: friendly_jacek

One more thing: how come the 18 wheelers are still using 15W40 and not 5W20?

That's an easy one: their engines are designed for oils which have 3.5 min HTHS viscosity. There's a reason they're called heavy duty engines. They work hard all the time. And if you have ever looked at bearings that come out of an engine with 600,000 miles on it, you would understand why thin oil is just not used.

18 Wheelers can use oils from 0W30 to 15W40. The exact temp ranges vary by manufacturer. Detroits for instance are not suppose to use 15W40 below 14F or 30 weights above 95F.