Rick's Free Auto Repair Advice

Choosing the right motor oil

This article is about choosing the right motor oil.

Do you know how to choose the right motor oil for your vehicle?

It’s  just one in a series. If you’d like to know more about oil click on the links below.

Choosing the right motor oil

Chrysler’s 3.5-liter engine has oil sludge issues

High mileage oil and oil filters

Oil Viscosity Warning

Synthetic Oil Facts and Myths

Synthetic oil versus conventional oil

What motor oil to use

What oil brand is best

What oil should you use

When to change oil

Best Oil filter

I volunteer in a lot of automotive forums and I constantly see people recommending motor oil viscosity grades that run counter to the manufacturer’s recommendations. That may have worked in the old days. But with modern engines you’re playing with fire and setting yourself up for premature engine failure if you use the wrong motor oil. You can also screw thing up by swapping in a different oil filters on the assumption that you’ll get better protection with a larger filter. It’s all part of a system and you can’t change just one piece without affecting the entire system.

First, let’s talk about motor oil viscosity and camshaft timing

Since this discussion started in a Ford forum with a 2005 Ford Taurus 3.0 Duratec engine, I’ll just use that as an example. The Ford spec calls for 5W-20 synthetic blend oil. This engine has variable valve timing VVT. VVT advances or retards the camshafts based on engine temperature, requested acceleration, and air/fuel feedback from the oxygen sensors.

camshaft phaser

New cam phaser

The cam “phasers” are hydraulic. The rotate the camshafts using oil pressure. The amount of rotation is controlled by a duty cycled solenoid that toggles on and off to regulate oil pressure to the phasers. However, the amount of duty cycle is a software algorithm that assumes you’ve used the correct oil. The programming figures duty cycle based upon the engine temperature and the expected viscosity of the oil at that temperature. So the solenoid is commanded to pulse “X” number of times per second to maintain proper camshaft rotation. **

With that in mind, the computer expects to see a certain level of “power contribution” from each cylinder. The computer commands a certain fuel charge based on requested power, engine temp, ambient temperature and barometric pressure, and camshaft timing, and then checks the crankshaft position sensor after each cylinder fire to determine if it got the expected power contribution, based on the rate of crankshaft rotation. If it see a slower rotation than it expected, one of the strategies the computer employs is to alter camshaft timing to see if the VVT is at fault. Incorrect oil viscosity is one of the factors that can affect the VVT mechanism’s ability to respond to computer commands. So substituting your judgment for motor oil against the factory recommendation can result in a DTC for VVT. It happens far more often than you think.

Oil viscosity affects timing belt and chain tensioners

Just like cam phasers, some timing belt and timing chain tensioners rely on a hydraulic piston to operate the tensioner. Using the wrong viscosity oil can affect timing belt and chain operation

Next, let’s consider the effect of altered motor oil viscosity on cold starting performance

Let’s use the same 2005 Ford Taurus. The Ford spec calls for a 5W-20 synthetic blend motor oil. I’ve pulled up the specs for Valvoline’s Durablend synthetic blend motor oil because it meets Ford spec WSS-M2C153-H. But you can use any brand that meets that spec.

At 40°C the oil viscosity for 5W-20 is 45.78(cSt). However, a 10W-30 Durablend at the same 40°C temperature has a viscosity of 71.45(cSt).

when to change oil

Cold starts reduce oil life

That’s an increase of 56%! Since the majority of engine wear occurs at cold startup, changing from 5W-20 to 10W-30 causes a significant decrease in flow rate. So you experience more cold engine wear. That factor alone should convince you not to vary from manufacturer’s recommendations. But it gets worse.

Now let’s talk about Viscosity Index, or the change in the oil from one temperature to the next. Here’s an official definition of VI from CASTROL:

“The VI of an oil is calculated from its kinematic viscosities at 40°C and 100°C and the resultant value indicates the temperature/viscosity relationship of that oil. An oil with a high VI experiences smaller decreases in viscosity as the temperature at which it is working increases than does an oil with a lower VI. Conversely as the ambient temperature decreases, the viscosity of a high VI oil will increase at a lower rate than that of an oil with a lower VI.”

So you want an oil with a higher VI

Now let’s go back to the Valvoline Durablend and look at VI comparisons. Durablend 5W-20 has a VI of 161, while Durablend 10W-30 has a VI of 141. If you switch to 10W-30, you’ll actually get 12.4% LESS VI than sticking with the factory recommendation.

Now let’s talk motor oil specifications

As I mentioned earlier, Ford’s recommendation for the 2005 3.0 Duratec engine is 5W-20. However, the manual also says the oil must meet Ford specification WSS-M2C153-H. If you look at the Valvoline Durablend specifications, you’ll see that only the 5W-20 product meets Ford’s WSS-M2C153-H spec. Even though Valvoline makes other viscosities in the Durablend line, none of them meet the FORD spec. That’s not surprising since Ford most likely wrote that spec specifically for 5W-20. But that’s precisely my point. The engine was designed and built for that oil. So were the camshaft phasers.

Which leads to the issue of motor oil viscosity index improvers

I’m going to generalize here because of the conflicting definitions of synthetic versus traditional mineral oil. It used to be that only Group IV PAO motor oil qualified as synthetic. But refining methods have improved to the point where high end Group III motor oils come close to the performance of Group IV oils and can now be called synthetic. Some manufacturers sell Group III as synthetic, others sell a blend of Group III and Group IV, and a few others only sell Group IV as synthetic.

Given that, let’s take a look at how multi-viscosity oil works. You can take a 10 weight oil and add pour point depressants to get that oil to behave like a 5 weight oil. You can also add viscosity index improvers (VII) to make it act like a 30 weight oil when hot. Or, you can take a Group IV 5W, forget the no pour point depressants because and flows when cold (Group IV oil doesn’t have parafin), and just add viscosity improvers to make it behave like a 20 weight oil when hot. The truth is manufacturers use both methods. So you might have a blend of Group III and Group IV oils with a minimum of pour point depressant additives and the minimum amount of VII additive.

Now let’s talk about 5W-20 and 10W-30 in the Ford engine. Whichever method the manufacturer chooses—whether they add pour point depressants or VII, the point is that 10W-30 requires more of those additives than the 5W-20 product because the viscosity range of the 10W-30 is greater (15 for the 5W-20 versus 20 for the 10W-30). Yet, if we go back to the Durablend specs, we can’t forget that the 10W-30 actually had a lower VI than the 5W-20. In other words, more additives gets you less VI. Keep that in mind.

Viscosity Index Improvers

I’m pretty sure you guys know what VII is. But for those who don’t, it’s a polymeric molecule that literally uncoils as it heats up. In lay terms it’s like adding flour to gravy to thicken it up, except that VII goes back to its coiled (smaller) state when it cools. To span a greater viscosity range, the oil formulators use a higher molecular weight VII. The denser molecule uncoils more than a less dense VII and that’s what gives it a larger range. But all VII additives are susceptible to shear. In high torque situations like heavy acceleration or hauling heavy loads, the VII gets cut like string beans in a blender. The more they’re cut, the less effective they are in maintaining the correct viscosity. So, if you substitute 10W-30 for 5W-20, you’re simply going to find yourself with a 10W-20 motor oil in short order. Worse yet, that oil has lower film thickness which is one of the most important features in a hot oil.

Chevron is an industry supplier of VII. Here’s their description of what happens to VII when subjected to shear.

“During routine engine operation and through continued use, engine oils are exposed to shearing mechanisms that break down the polymer molecules, reducing the oils molecular weight. This can lead to viscosity loss and a subsequent decrease in oil film thickness. If too severe, this can cause undesired friction and engine wear with oils that are not formulated with the proper viscosity characteristics.

Molecular shearing of lubricating oils typically occurs in very tight spaces between moving parts that are in close proximity to one another. Bearings and ring faces are classic examples of where engine oil shearing can occur. Polymer coils also react to changes in shear or flow-rate in the oil.

Under more extreme (very high) shear conditions, the coil can be pulled apart and polymer chain broken into two smaller chains. These two smaller chains have less impact on the oil’s viscosity than a single large chain, so the viscosity falls in this case as well, especially since the chains also align in the direction of flow. When the shear is removed, the broken polymer chain cannot re-form into the single large chain because the coil has been physically and chemically changed. Consequently, the oil’s viscosity does not return to its original value, but remains at a lower viscosity. This is known as “permanent viscosity loss.

Note that larger coils are subject to larger forces when the oil is exposed to shearing stresses. This is because a larger coil has more “surface” over which the stretching forces of the oil’s viscosity gradient can operate. Larger coils are therefore more easily pulled apart and broken than smaller coils, so therefore, high molecular weight molecules are more vulnerable to permanent viscosity loss.”

Now that you understand the concept of VII shear and shear stability, reconsider the theme of this article; Can you switch from Ford’s recommended WSS-M2C153-H specified 5W-20 to 10W-30 motor oil and still have the same protection? You should assume that Ford calculated that spec to anticipate a set amount of viscosity loss and film thickness due to 5W-20’s shear stability. But to achieve 10W-30, you should also assume that the manufacturer uses a higher molecular weight VII and thus has DECREASED levels of shear stability and LOWER film thickness.

So you thought you were doing right by your engine by using a “heavier” weight oil. But what you might be getting is LESS wear protection and lower film thickness than you would have by sticking with the recommended 5W-20.

All of which is why I don’t recommend varying from the manufacturer’s recommendations. You actually get less protection with a higher weigh oil.

** In a way, this is similar to the GM’s new oil life monitoring system. By requiring the use of DEXOS oil the oil life system is working from a known starting point. Because GM writes the specs for the oil, it knows the exact type and amount of each additive, especially the anti-wear and anti-oxidants. Since the oil life monitoring systems tracks the engine and ambient temperatures at each startup and knows your driving habits, including torque and load, it can calculate the life of the oil. Longer drives at lighter loads gets you longer intervals between oil changes. But lots of cold starts and short trips uses up the wear, oxidation, corrosion inhibiting and dispersant additives. Some people think DEXOS is a rip off. But the flip side of this approach is guesstimating whether you actually need an oil change. How else would an oil life monitoring system work if it didn’t know the composition of the oil?

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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