Thin vs Thick Discussion, Chapter 4

Sarasota, Florida
Motor Oil 104
Part Four. It is not what we thought.

Now let us finish talking about the differences of mineral verses synthetic oils. I will compare the same weigh or grade of oils showing that the operating viscosities are the same whereas the startup viscosities vary:

Mineral oil:

Oil type...Thickness at 75 F... at 212 302 F

Straight 30..........250....................10..........3
0W-30..........There are probably no pure mineral based motor oils in this range......

Synthetic oil:

Oil type...Thickness at 75 F... at 212 302 F

Straight 30...........100...................10...........3

Since most synthetic oils thicken less on shutdown your startup will be easier and so will the stress on your engine. This is perhaps the best thing the synthetic class has over the mineral based oils. When I look to choose an oil I first look for the appropriate viscosity at operating temperatures. I then look at the cold ( 75 F here in Florida ) start up viscosity. It should be least distant from the full operating temperature.

In the old, early days of synthetic oils there were often leaking engine seals. This is no longer a problem. People sometimes use a thicker oil to minimize gasket leaks. This seems obvious to me. Repair the gasket. Do not hurt or Stress your engine with an oil that is too thick for proper function.

Some people have said they use thicker oils because they only use their cars every 2, 3 or 4 weeks. They are afraid that thin oils and mineral oils will fall off the engine parts and result in a lack of lubrication at startup. Think about your lawn mower over the winter. It gets gummed up solid. The oil and fuel thicken over time resulting in engine failure. It is said that synthetic oils cling better and longer to parts inside your engine. Anyway, oil on the surface of parts does not lubricate. It is the FLOW of oil between parts that lubricates. Thick, old, waxy oil can only be bad.

As it turns out synthetic oils do cling to parts better as they have higher film strength than mineral oils. Synthetics are thinner overall. They have greater slipperiness. Yet they stick better to engine parts. Again, this concept is the opposite of normal thinking.

I have seen several car owner manuals that are now stating that oils do not need to be changed but every 10,000 miles or more. The same manual also states OR every 12 months, whichever occurs first. My feeling is that you can probably go 5,000 miles on the average (in a sports car) but you must change your oil in the spring time at a minimum, particularly up north. Oils form waxes in icy cold weather. There is a permanent thickening of the oil. It is because of this thickening that they have you change oil yearly, even if you put little or no miles on your car. I have kept oil in some engines for up to 3 years here in tropical south Florida simply because my vehicles do not experience temperatures below 50F.

I truly believe that oil is much better being a little too thin than too thick. Over the years we have been going to thinner and thinner oils despite hotter engines with turbos and the like. The tendency is that people figure they need a 40 grade oil but then use a 50 instead. Better thinking is that if you think you need a 40, use a 30 grade oil instead. I firmly believe this based on my experience and all I know about oils.

One way of measuring the thickness of moving oil is in centiStokes or cSt. Most engines want the oil viscosity to be around 10 cSt at normal operating temperature. The really thick multi-grade oils have a viscosity of 20 cSt at operating temperature. One is not twice as thick as the other, it is only 10 cSt thicker.

As we increase the heat from 212 F to 302 F the most commonly recommended oil thins from 10 cSt to 3 cSt. The thicker oil drops from 20 cSt to 4 cSt. Note that in a very hot engine the difference between the two oils is now only 1 - 2 cSt. In other words they have about the same thickness. There is little advantage to a thicker based oil as a 20W-50 at very high temperatures. No, the 4 cSt oil is not twice as thick as the 2 or 3 cSt oil. This difference is almost insignificant. For these constantly higher temperatures synthetic oils handle it better than mineral based oils.

There is an advantage of using the thinner, 10W-30 during the 20-30 minute startup period where 90 percent of the engine wear occurs. At 75 F the thicker oil has a viscosity in the range of 250 cSt while the thinner oil has a viscosity of 100 cSt. The thicker stuff is 150 cSt thicker. This is a big difference. I am using the 20W-50 as my thicker oil example here.

People are always asking about using additives into the oil tank. I believe you should not do this. The oil companies and engine manufacturers work together very hard to give you the product you need. Engines are running hotter, longer with more BHP from less CID. Smaller, more efficient engines are getting us more MPG and yet better acceleration. These engines last longer and are more reliable.

Part of that reason is the nature of the lubricants. There is a lot of competition to get us the best working motor oil. Independent additives cannot make the oil better and may make things worse. There have been engine failures as a result of adding some of these aftermarket products to motor oil. The same may be true for aftermarket air filters that promise more HP and MPG. Often these filters allow more dirt into your engine and this hurts the motors.

Motor oil that is labeled for RACING ONLY (almost always synthetic) is not usable for every day driving. Often these have more additives that are toxic to your catalytic converters and the environment. These oils generally do not have detergents. These are very important for your engine unless you plan on taking it apart every few weeks and cleaning every single surface. The oils do not meet the API / SAE requirements for the current ratings.

You do not need to use the exact engine oil type and brand that your car manual tells you to use. Oils are pretty general. They are not that different. Ferrari was married to Shell. If you call them up and ask to use Valvoline instead they will tell you that they have not tested that brand in their cars. They only tested the engine with Shell oils. They cannot comment on the performance of other oils in their engines. This is a fair statement. The reality is that the Shell and Valvoline oils of the same specification (viscosity, API and SAE ratings, synthetic or not) are likely very similar.

Our 2017 Bentley Flying Spur Speed W12s says to only use synthetic Mobil 1 0W-40 grade oil in the engine. I use mineral based Pennzoil 0W-20, a grade I have used in every Supercar we have ever owned. But I am not driving the Autobahn at 200 MPH. ‘The only 4 door sedan capable of such speeds.

People often say that their old 1980 car manual says to use a specific Brand-X motor oil. They keep trying to locate these older oils. First, just about any oil brand that meets the current specifications will do. Second, all oils are much, much better now. They are all much better. One could say that synthetic oils are better than mineral oils but it is hard to say that one brand is that much better than any other. Personally, I do stick to the big names. It does not mean that small motor oil companies are not as good. They could be better for all I know. And modern specified oils are back specified for all oils that came before them.

In general the synthetic lubricated engine will turn over easier. This has the effect of using less power from your starter motor. It will last longer. Your battery has less of a current draw. This will also last longer. The battery was discharged less during the start so the alternator will rob less power from your engine to recharge. The alternator lasts longer and you get a little better fuel economy. The only downside of synthetic lubricants is the cost. They cost more than mineral based oils.

So in conclusion synthetic oils are more costly but may offer a little bit better performance and wear resistance for your engine. But the gap is less today than in the past . Often mineral based oils have a component of synthetics and synthetic oils may in fact have mineral based mixtures.
There is an advantage of using the thinner, 10W-30 during the 20-30 minute startup period where 90 percent of the engine wear occurs. At 75 F the thicker oil has a viscosity in the range of 250 cSt while the thinner oil has a viscosity of 100 cSt. The thicker stuff is 150 cSt thicker. This is a big difference. I am using the 20W-50 as my thicker oil example here.
At 75F even a 20W-50 is going to have 100% pumpability and flow to all parts of the oiling system, and provide adequate lubrication. Where's the testing data that shows more wear is occurring at 75F start-ups if you're not using 0W-20 motor oil?

You have to start getting way colder than 75F to start talking about "lack of lubrication" on a cold start-up vs the rated oil viscosity. Lots of motorcycles use 20W-50, and the lowest temperature for use shown in manuals for 20W-50 is around 15F (-10C). And yes, I've compared many motorcycle and car engine service manuals to compare the rod/crank/camshaft clearances, and they are all just as tight.