Straight Weight vs. Multigrade

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Staff member
Jun 5, 2002
Iowegia - USA
An SAE paper in 1991, (910742) was entitled,
"Direct Observation of the Friction Reduction of Multigrade Lubricants"
by some people at MIT.

Using laser fluorescence and other methods, they measured lubricant film thickness
of the top piston ring in a running engine using 5 dino oils; two single grade, and three multi-grades.

The engine was a small IDI diesel.

The report said that multi-grades had lower friction than straight-grades by about 20% for the top ring .
Did they give any theroy on why this was so?

I've always thought that a straight weight oil had less, or no VI Improvers and so would have better friction reducing ability at operating temperature, at the expense of cold flow.
This would be true as the 30wt will not shear back in a high shear zone such as at the rings because it is a newtonian fluid with no vi's where as a multiviscosity oil will shear down to the lower number on the oil since the vi's will collapse or compress in a high shear zone therefore able to get into areas such as around the rings and such. So when you experience oil blowby, then move to a straight wt oil where it will reduce its ability to get by the rings thus reduce oil blowby.

The reason given in the paper was that the top ring was wetted less with multivis than with straight grade. Now before anyone gets excited, let me explain ring "wetting."

Visualize the piston and cylinder on its side, with the ring just a few microns from the cylinder wall. Zoom in to just the top ring and the cylinder wall. Assume the end of the ring has a spherical shape to it where it contacts the cylinder wall.

As the piston moves up lets say on compression, or down from combustion, there is a wall of oil on both sides of the ring. The height of this wall is called the "wetting" height. For straight grades, this wall is higher, thus the ring is moving a larger volume of oil and moving against a high wall of oil heats it up locally, and you have viscous drag, thus robbing the piston of some HP (albeit very small). This presents a higher level of friction - viscous friction. (Any resistance while in motion is kinetic friction).

Now the multivis still provides plenting of wetting for the ring, but the wall or "wetting" height is lower, thus less oil is moved, and we have less viscous friction.
GW: "Would you say that one would see lower wear and higher oil consumption with a straight grade?"

I know this wasn't directed at me but I'd say lower wear, lower oil consumption, slightly higher fuel consumption and slightly less horsepower from using straight grades (i.e., 30 instead of 10W30).

there is a wall of oil on both sides of the ring. The height of this wall is called the "wetting" height. For straight grades, this wall is higher, thus the ring is moving a larger volume of oil and moving against a high wall of oil heats it up locally, and you have viscous drag, thus robbing the piston of some HP (albeit very small). This presents a higher level of friction - viscous friction.

Interesting observation but lets look at this in a mechanical way.... If the flow properties of both oils at temp are 30wt, then the viscous friction as reffered to, would or should be near the same except..., Where the difference seems to be is that when the rings go up, both oils will be coating the same region of the cyl wall since both are being mechanically pumped and dispursed across the same region on the wall since both are essentally a 30wt, but when the pistion comes down, the straight 30 cannot shear down to the lower base wt as its base wt is st 30, thus providing the drag, whereas the muti visc oil will shear back to its lower base wt and leaves a small film in the groves or crosshatch pattern that is scored into the cyl wall when built, therefore allowing the piston a small amount of lubrication for the rings to run on. This is one reason why you don't see that many engines having to be rebuilt leaving a ridge from where the top ring wore down the cyl wall as it use to with straight wt oils.

Any comments to this theory?

[ June 14, 2002, 09:29 AM: Message edited by: BOBISTHEOILGUY ]
Thanks Bror,

Are you basing this on your experience? I have been trying to find as much info. as possible on this and it seems that conventional theory is, as Widman pointed out in the other post, that monogrades will allow for higher oil consumption.

Another area where monogrades apparently fall behind is in high temperature protection. Multi's have a higher V.I. Index than mono's, therefore they can resist thinning better as temperatures climb above 100C. Widman did also point this out, but unfortunately his link to a chart that shows this did not work for me.

But....all the comparisons I find were made between petroleum oils. Synthetic oils, including synthetic monogrades, have inherently higher V.I. indices, and a really stubborn guy like myself might just find that these shortcomings really only apply only to petroleum monogrades. (?)

I plan to give this a go and to compare oil analysis results and oil consumption rates with the results that I have using a multigrade in my high-miler.

[ June 14, 2002, 09:50 AM: Message edited by: GW ]
I think Bob is giving a "rheological" explanation while I was giving a "kinetic"
explanation and essentially we are saying the same thing. I was attempting to amplify
what the paper gave as an explanation, using the example given.

The oil film is actually squeezed to one side or the other (tangential to motion) and "wedged" as the piston ring along the cylinder liner. The "wedge" of oil is thinner near the ring and thicker away from the ring. The actual film thickness between the rings varies with crankshaft angle and with ring position. For the oil ring, the thickness of the film averages about 0.25 microns (0.25 millionths of a meter), while the top and second ring film thickness averages about 1 micron. Thats for all lubrication regimes (hydrodynamic, boundary, etc). The reason for the thinner film in the oil control rings is because its tension is usually higher than the other two rings. Newer type rings have taper-faced or barrel-faced shapes to control oil consumption and different profiles to reduce circumferential tension (less ring friction).

If you observe a top ring with a any face shape (spherical, elliptical, barrel, taper), you will see a definite wedging effect of the oil as the ring traverses the cylinder. Again, the height of the wall of oil is definitely higher for single grade, than for multigrade and friction is 20% higer for straight grade than for multigrade.
GW and BROR,

Bror I think sums up the net effect.

I would think synthetics, because of their reduced friction, would offer lower power losses
in the ring pack and bearings.

GW, I would be interested in your results for straight weight synthetics vs. multi's.
A lot of theories to try to explain it, so here is another question. Truck makers do not have to comply with mileage standards. They sell the big things based on how many million miles they can run with the big guys on long haul and delivery routes.
Volvo won't recomend any thing but 15w40, Scania, Mack, Caterpillar, and Cummins the same. Why?
I've seen the videos of the Volvo VDS 2 examination. 320,000 miles of heavy work with 40,000 mile drain intervals. Take the engine apart and can't have that ridge on the sleeves, nor other signs of wear.
If there is any truth to lower wear with single grades, why don't they use them?
It's been estimated that 80% of engine wear is caused at startup. If the multi-weight has better pumpability and reaches the engine lube points faster, then it should reduce wear over a thicker oil that takes longer to pump. I think once an engine is running at cruise, one oil is as good as another, as long as it maintains the critical film between the metal parts.
GW, that's based both on what I know and my anecdotal experience. The problem is, I tend to experiment with oil weights and brands on small engines and fuel economy isn't really measured, just consumption. And, I sometimes found that consumption in those single cylinders would cease if synthetic was used (old Mobil 1) ... even in older engines that had been running straight 40.

I follow the mileage in my little Honda Civic like a hawk, however. The worst mileage I think I ever got with the thing during the middle of summer was with Red Line 10W30. That stuff was noticeably thicker than the 5W30 and 10W30 Mobil 1 and Valvoline Synpower I had been using before that.

The lightest oil I had ever used in the Civic was Mobil 1 0W30 but I used it in the winter when my mileage normally drops off because of the cold. It would be interesting to use this stuff or one of the 5W20 oils in the summer to see the effect on mileage, but I won't use this super-thin stuff in the heat.

I also saw a dyno test of a Ford Mustang. They used a mineral oil and Red Line synthetic, both 10W30. The Red Line was the winner through the first 2/3 of the rev range (if my memory's accurate) but then the dino oil began to edge into the lead as the engine approached redline. I suspect that the Red Line was more stable at higher RPMs and the dino oil had started to thin by that time and produce less drag (but probably allowed more wear). They offered no explanation themselves.

I used a straight 30 in my 1986 Ford Escort GT for a fast trip to Montreal and my mileage suffered slightly, but not too much. It was too long ago.
O.K. guys, I am not saying you are wrong and I do respect your opinion but...... this is all conventional theory with conventional oils. Please show me some test results and comparisons with synthetic monogrades - they do behave somewhat differently do they not?

GW, I would be interested in your results for straight weight synthetics vs. multi's.


The experiment has begun - the SAE 30 is now in use. The only thing I have noticed is a slight reduction in oil pressure both at idle and at 3000 RPM. I will post analysis results after 10,000 KM.

Just to fill you in on this experiment.....I now have 7000 KM on this oil and it is quite clear that the oil consumption has increased but only when the vehicle is used for extended periods in the city. (we do not get there that often)

In the first 3000 KM I was down one litre of oil. (as compared with a consumption rate previously of 1 litre/10,000 KM using 10W40 synthetic) During this period the vehicle did get to the city for about a week of daily driving)

In the next 3000 KM, the vehicle saw only our regular rural/highway driving and the engine had used almost no oil, (down just a little on the stick) It was only when the vehicle did another stint in the city for a week that it used almost another litre of oil.

I will be sampling at 10,000 KM to compare wear rates etc. with previous analyses on the 10w40. (vehicle now has 388,000 KM on it)

Fuel economy has not been affected with the use of this oil.

What do you make of this?


[ August 28, 2002, 01:19 AM: Message edited by: con carne ]
One might try and keep the vehicle out of the city. (HeHe!)

Seriously, the only answer I can come up with is that in stop-and-go city traffic your engine vacuum is on the average higher as well as the temps so the oil is taking a greater beating. I don't know the temps or the driving habits in Argentina, but up here that kind of driving is considered severe service.

What synth oil are you using? Most full synth oils have lower volatility than comparable dino oils, even if straight weights.
Another interesting tid-bit to this subject:
"The Basics of Multigrades
From "The Practical Handbook of Machinery Lubrication":

Multigrade oils are made by blending a low-viscosity oil with special additives called Viscosity Index Improvers. For example, when these polymer additives are blended in the correct proportion with an SAE 15W oil, the oil flows like an SAE 15W oil at low temperatures and like an SAE 40 oil at high temperatures. The result is an SAE 15W/40 oil that will provide wide protection over an extended temperature range.

Multigrade oils have five main advantages over single grade oils. These oils offer:

(a) One oil for year-round use.

(b) Improved low-temperature starting.

(c) Excellent high-temperature performance.

(d) Improved overall fuel economy by providing less battery drain on cold starts, less idling time and faster warm up.

(e) Faster, full-pressure lubrication over a wider temperature range. That is, an SAE 15W/40 oil provides full flow lubrication in approximately 1 minute, and 45 seconds at -25°C compared to an SAE 30 oil which requires approximately 3 minutes and 15 seconds to achieve full flow lubrication."

Not that any of us would use SAE 30 at -25C.
The synthetic that I am using is Amsoil SAE 30. The 10W40 that I was referring to was Amsoil's as well.

I agree with you on the vacuum theory and I thought that I might try replacing the valve seals on this high-miler. The interesting thing is that I did not see this problem with the 10W40 so unless this is a new engine problem, these oils are definitely behaving differently in my engine.

Widman, I agree also with everything that you have said, but I thought I would try running this monograde synthetic to see if what you had said applies - synthetic monogrades act more like multigrade than straight-weight petroleum oils do. What you have quoted in your post is what I believe to be conventional theory for petroleum oils.

[ August 28, 2002, 05:01 PM: Message edited by: con carne ]
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