Automotive Lubricants

The Hidden Side of Viscosity


The Hidden Side of Viscosity
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It seems like forever—and maybe it has been—that I’ve heard it said that the most important property of any lubricant is its viscosity. Quoting from the 1955 Industrial Lubrication Practice by Paul D. Hobson, viscosity “is,  for almost all practical applications, the most important single characteristic of an oil.” 

This was true nearly 75 years ago and even way before that. So what is viscosity?

The general definition is resistance to flow. That resistance is the friction between the layers of molecules in the oil itself. since petroleum-based oil is a mix of many different molecules (structure and size). Synthetics such as PAO have a much more defined chemistry, so the size of molecules has more impact on viscosity.

Steve Haffner’s excellent article in the October issue of Lubes’n’Greases pointed to the fact that we’re getting pretty close to the bottom of the viscosity well. The use of very low-viscosity engine oils to reduce fuel consumption and lower emissions may be getting close to the limit available from changes to viscosity alone. If you haven’t read it before, I recommend that you do so.

As most all of you are aware, viscosity of engine oils has been dropping for quite some time. When I started in this business, the dominant viscosity grade for engine oils was SAE 30. There were SAE 10-40 products in the marketplace, but their primary selling point was that they were good for use in the winter as well as the summer. There were questions about the cost of crude and its impact on gasoline prices. Incidentally, I was paying $0.25 per gallon for regular gasoline. In addition, I was getting a 10% discount, so my net cost for gasoline was $0.225/gallon.

That all changed when the first Arab oil embargo occurred in the early 1970s when there was gasoline rationing (maximum gallons that could be purchased and restrictions on when you could even buy). The price jumped to $0.75/gallon. Essentially, this represented a 300% increase over the mid-1960s price. The race to develop fuel savings was on!

First up was simply detuning the engines. That was totally unacceptable. Next up was downsizing the engines. This was better, but fuel efficiency wasn’t very good. Carburetors just weren’t capable of regulating fuel consumption effectively. There was an early attempt to improve friction characteristics with friction modifiers, but it soon became clear that decreasing viscosity was the most effective way to reduce fuel consumption.

Starting in 1980, I began tracking oil viscosity. At the time, the National Petroleum Refiners Association (NPRA) was tracking engine oil sales by viscosity grade. I took the data and created a pseudo-formulation for each viscosity grade using a standard additive treatment and VI additive. From that, I could calculate a base oil combination and create a map of base oil blends. The numbers are very revealing. From 1980 to 2010, pool base oil viscosity dropped from 7.5 centistokes to 5.5 cSt at 100 degrees Celsius. That trend has continued.

Separately, the mix of vehicles in the market (VIO) is undergoing changes as well. Jim Lang’s market report points out that the age of VIO has grown to the point that the average age is now 12 or more years. Obviously, that average is not a nice, simple-to-analyze bell curve. There are more, newer vehicles, but I really doubt that my old 2004 Nissan Quest would do well on SAE 0W-20 engine oil. In fact, with the average age of vehicles above 12 years and the trend to drive vehicles longer, there is likely to be an increase in engine oil viscosity to service the beloved 2005 chariot in your garage. 

Another driving force for old vehicles is the ever-rising cost of new vehicles. My car experience with my last two is educational. In 2020, I leased a Honda Pilot, which is the upper end of Honda’s SUV line. Don’t ask me why I did that. Suffice to say, I liked the look and the performance of the vehicle. Three years later when the lease expired, I turned that one in and replaced it with a Honda CR-V, which is a mid-range SUV. I am paying slightly more for the CR-V; the down payment is slightly higher, but again, I liked the look of it and the “bells and whistles” that came with it. In retrospect, I probably should have stayed with the Pilot.

So here’s the situation: I have an expensive vehicle, which will get me anywhere I want to go and with good fuel economy. I had an expensive vehicle (but slightly less fuel efficient) that would take me anywhere I wanted to go. 

This brings us back to the original discussion: Will viscosity continue on its downward trajectory, will it stabilize or will it start to climb? 

Here’s where I stand on the subject: I think it will stabilize to slightly increase until 2030. Most vehicles on the road can do just fine with SAE 5W-30. There is plenty of history with that grade, and it can cover models dating back to the 1990s. SAE 0W-20 will cover the newer model years. Sad to say, the SAE 10W-30 is destined for the scrapheap of history. This will result in a much simpler engine oil inventory for garages and oil change stores.

But what about the specialty grades that are becoming all the rage? Will the SAE 20W-50 for your ATV be a new grade of oil, and will there be a big push? There are new lines of oils for specialized applications, which will satisfy the resistant oil buyer, but it probably won’t be in the bulk tank.

I haven’t addressed the last end of the viscosity issue. What are the OEMs going to do? They have very little desire to change oil categories, since they are investing their time and funds developing their BEV product lines. There will be pressure in some quarters to get at least one more engine oil category, but I think the OEMs will be dragging their feet on that one. Category development is an extremely expensive proposition, both in time and costs.

So here we are now: Oil viscosity has probably hit rock bottom, the VIOs are getting older and the needs of the engines are probably not going to change much. It seems to me that a status quo has been reached. Success will go to the oil brands and oil changers who can give their customers what they need and at a good price. Same as always.   

Steve Swedberg is an industry consultant with over 40 years experience in lubricants, most notably with Pennzoil and Chevron Oronite. He is a longtime member of the American Chemical Society, ASTM International and SAE International, where he was chairman of Technical Committee 1 on automotive engine oils. He can be reached at

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Automotive Lubricants    Finished Lubricants