My last two cars have had nothing in them except Mobil One synthetic.

On my TCH, which has had only 0W-20 in it, I just went throught the first oil change, and my first tank afterward showed a decrease of approximately 2 MPG over the same commute that I have gotten 44 MPG on since I got the car. The second tank after the oil change is now showing an FE improvement and I am back to 43 MPG.

My wife's Forester uses 5W-20, and during the last three oil changes I have noticed that the FE dropped from 28 MPG down to about 26 MPG for her commute. It came back up again after a refuel or two to what it was before.

I wonder if synthetic oil, or at least Mobil One, needs a to get hot before it gets stable -- maybe a "break in" period. Anyone else noticing anything like this?

It is the ignorant among us that will eventually destroy us all.

It is more accurate to say that oil viscosity tends to decrease as it is in service. The lower viscosity reduces 'pumping losses' but this is compensated for by the increasing particles in the oil that bridge the oil gap. These little, microscopic pieces of "stuff" add drag so the oil is not quite as slippery as it was.

I've got some charts showing what happens with transaxle oil:


The same appears to happen with engine oil.

Bob Wilson

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
Free speech, dialog and knowledge thrives without the poison of SPAM.

I can't comment on the break-in issue but have some questions for you.

My TCH is coming up to its first oil change. My dealer will provide the first one free. Did you do yours at the dealer? Will they put in 0w20? Also, do you know for sure that the oil in your new camry was indeed 0w20. I would be depressed if my mpg drop cause of a different oil grade. The location of the filter and plug is very accessible. I am however somewhat hesitant on doing it myself for warranty purposes.

If you are doing your own oil change, are you not concerned about warranty. I have seen a post that recommend buying the oil and filter from Toyota so that warranty denial can be avoided. That's just hard to fathom. Thoughts anyone?

Although there can be some competing factors pulling the viscosity in different ways, engine oil tends to become more viscous through its lifetime. I'm reminded of a study on engine oil life of Amsoil and Mobil 1 synthetic oils. In general, both of these samples' viscosities increased with their lifetime.

As for the competing factors, I pulled the following description from here:

Contamination from wear or debris increases the viscosity. Incomplete combustion, fuel leaks, coolant leaks, overheating, additive loss and detergent breakdown tend to decrease the oil viscosity before complete failure cooks the lubricant causing its viscosity to increase.

Interesting. I checked with the PriusChat oil spreadsheet and that data also shows a similar pattern of slowly increasing 40C viscosity.
I've not paid much attention to engine oils. I've pretty well adopted a spring/fall chance schedule with lighter winter viscosity oil and standard summer oil.

Thanks,
Bob Wilson

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
Free speech, dialog and knowledge thrives without the poison of SPAM.

This was/is my understanding also, ...which is why I ask the question in the first place. It makes no sense. If I were to graph FE based on the history of both vehicles from oil change to oil change, it would take an immediate [vertical line] dip of approx 1.8 MPG and then gradully come back to its original point in about 800 to 900 miles on the Subaru or 1000 to 1200 miles in the case of the TCH, and then it would remain essentually flat with very little degradation until the next oil change. It makes no sense unless either the oil or the filter are able to impact FE as nothing else changes. That would suggest that the degradation is not linear, but is a curve, and at some point on the curve, other factors become more important to FE than oil viscosity.

It is the ignorant among us that will eventually destroy us all.

Since you mentioned oil filter, let me throw one more variable into the thought process.

When you put a fresh filter on the car, the oil should be flowing through the media. Then as the oil starts to accumulate particulate, it starts to clog the filter, and the force required to push it through the media increases. Suddenly the force is greater than that required for the relief valve and the oil starts to bypass the media. Now that it has started to pypass the media, the relief valve does not require as much force to stay open, and the net result is it requires less force to go through the bypass route than it did in the new filter media resulting in less drag on the oil pump and better fuel economy.

Just a theory, but I really hope it is not true as it would mean the oil filters on most all cars don't really serve any useful purpose.

Nope. Where I work we can do a crude check for metal and for filtered materials in oil (we cannot check viscosity, however). Very low metals count of less than 0.01 PPM, no foreign matter for both vehicles. Certainly nothing that would open the bypass.

It is the ignorant among us that will eventually destroy us all.

Seconding fastmover's response to ag4ever. The oil filter bypass is an extreme case oh-shoot safety feature that will normally never open.

DAS

Not really the way it works. The oil filter circuit takes a small percentage(10%) of the oil flowing to the engine and filters that. As/IF the oil filter cloggs the bypass will open to allow all the oil into the engine. A full flow oil filter would be very large and could damage the engine if just slightly restricted. I have never seen a full flow system on an automotive application. This does over time clean the oil, but the quality of the filter is VERY important.

Most likely the oil does have a breakin period as some of the long HC chains will shear and lower the viscosity after a few hot/cold cycles. IIRC non petroleum base sythetics have less of a break in.

Ducit Amour Hybridae
Mark Smith
Master Certifited Technician
Tempus non reparabilis fugit
"It isn't pollution that's harming the environment. It's the impurities in our air and water that are doing it."

Best Tank 35.0 MPG