Thin vs Thick Discussion, Chapter 9

Sarasota, Florida
Motor Oil 109
Chapter nine. Let’s start over.

We have seen that 0W-30, 5W-30, 10W-30 and straight 30 grade oils all have the exact same viscosity at 212 and 302 F. What about startup viscosities? Do 0W-20, 0W-30 , and 0W-40 all have the same viscosity at a 75 F startup. The answer is no. The SAE J300 standard allows for this discrepancy. Here are some examples:

..Viscosity at 75 F startup..


The numbers are not exact but they show clearly that the ”0” represents different startup viscosities. This is unlike the 0W-30, 5W-30, 10W-30 and straight 30 grade oils that all have the exact same viscosity in a Hot engine = 10 cSt.

I would like to comment on the following statements made by a knowledgeable automotive enthusiast:
“Pressure and flow are tied together with viscosity, but none have anything to do with lubrication. Lubrication is a property of the fluid, not the force. The oil pump would pump water just as well, but it would offer no real lubrication. If we double the pressure, we double the flow. If you decrease the viscosity to a lighter oil, you increase flow at a loss of pressure. High flow helps to carry away more heat. High pressure helps to keep metal parts like the bearings out of contact with each other (scuffing).”

Here is one example. Take an air conditioner closed bearing compressor for your home’s A/C unit. Put a nipple on the bearing at one end of the shaft. Now pressurize the bearing. It will do nothing extra to reduce wear, nothing. As long as the flow remained the same in your automotive engine you can increase pressure any amount you want but this will not improve lubrication. It is a function of the lubricant itself and the flow.

I give you the following example to help visualize what is happening with motor oil flow. This assumes the oil has no internal resistance. In actuality doubling the pressure will not double the flow but will be slightly less. And thicker oils have more resistance than thinner oils for all situations. But simplified we get the following in a Ferrari:

For a 30 grade oil at normal operating temperature:
1,000......20 PSI....1
2,000......40 PSI....2
4,000......80 PSI....4
8,000... 160 PSI....8 BUT, the maximum flow because of the oil pop off relief valve at 90 PSI will be 5. The presssure in the Ferrari does not go past 90 PSI even when the RPM increases. As there is no increase in pressure there cannot be a further increase in flow.

For a 30 grade oil at operating temperature
and a higher output oil pump:
1,000......30 PSI....1.5
2,000......60 PSI....3
4,000....120 PSI....6 The maximum flow because of the oil pop off valve at 90 PSI will be 5
8,000... 240 PSI....12

Let us compare a 40 grade oil at operating temperature:
The oil is thicker, has more internal resistance and therefore requires more pressure to get the same flow (baseline engine).

1,000......30 PSI....1
2,000......60 PSI....2
4,000....120 PSI....4 The maximum flow because of the oil pop off valve at 90 PSI will be 3
8,000....240 PSI....8

For a 40 grade oil at operating temperature
and a higher output oil pump:
1,000......45 PSI....1.5
2,000......90 PSI....3 The maximum flow because of the oil pop off valve at 90 PSI will be 3
4,000....180 PSI....6
8,000... 360 PSI....12

Increasing the pressure while using a higher output pump will increase the oil flow but increasing the pressure by increasing the oil thickness alone will result in less flow. It takes more pressure to move a thicker oil. When you go to a thicker oil the pressure goes up because of the increased resistance, and therefore reduction of flow. Because the pressure is higher sooner, the relief valve cuts in sooner. Flow will actually be less when the RPM is up and the flow is needed the most.

There is more to these graphs, but I will continue with the next chapter.

Furthermore, in review, pressure does not equal lubrication. Let us look again at a single closed “lifetime lubricated” bearing. We could hook up a system to pressurize the bearing. This can actually be done. We could have the oil at ambient pressure. We could then double, triple, quadruple the pressure of the oil. The oil is non-compressible. Regardless of the pressure we would have the exact same lubrication, that of the ambient pressure lubrication.

The physics of lubrication as I said earlier show a relationship of flow to separation pressure. Lubrication itself in automotive engines is pressure independent.

Even water can be used as a lubricant. This is partly because of its high surface tension. It is used in many medical devices and other systems that are under or exposed to water. It is just that water rusts metal parts making this unsuitable for automotive engines. It actually has a higher specific heat than oil. It can therefore carry away more heat than oil from bearing surfaces. In this respect water is a better lubricant than oil. Many high performance cars have oil coolers to aid in this function of motor oils.

Another difficult concept: Many say that the oil flow is viscosity independent as pumps usually function by displacement of fixed amounts. So long as the relief valve does not come into play this is true. Unfortunately, as I have shown, thicker and thicker oil results in earlier oil bypass so there can be no increase in flow despite an increase in pump RPM.

In every car I have owned the pressure increases in a linear scale with RPM. This means the flow also increases in a linear fashion. The pressures have then suddenly Stopped increasing further though the RPM keeps increasing. In the Ferrari, the maximum pressure is 90 PSI. The pressure readily rises to this level but does not increase past this point at all. It does not go to 100, 120, 150 PSI as one might think as the RPM is still increasing. No. The pressure is stop limited at 90 PSI.

Since there can only be an increase in flow if there is an increase in pressure there cannot be any additional flow. And we NEED more flow with increasing RPM and stress on the engine.

Engine wear is often tested in engines that are run at full RPM and under full load after being fully warmed up. Other tests are bench top laboratory tests. Under these conditions there is evidence that thicker oils and even single grade oils show less wear. Even then, the wear is often not that different in thinner or multi-grade oils.

In oil testing done in the cars as they are used every day the story is different. Much of the wear occurs during the 30 minutes it takes oil to warm up fully. After this, wear is near zero, even with the thinnest of oils. I believe the difference between mineral and synthetic oils today is less than in the past. Yet in my high energy engines I do prefer the top brand fully synthetic motor oils.

Finally, air filtration for your car. When we talk about wear in our engines we must talk about air filtration. Oil gets dirty when particulates accumulate. Some say that dirt is more of an issue with thinner oils. In any event, I change my air filter twice as often as required by the owners manual even though I do not drive in dusty conditions.



Staff member
Indianapolis, IN
... In any event, I change my air filter twice as often as required by the owners manual even though I do not drive in dusty conditions.

I cannot disagree with this philosophy more. Frequent air filter changes are not only wasteful, but likely harmful. There are many sources of information (such as our member Jim Allen's trip to Fram's testing labs), as well as SAE studies such as Donaldson's "Total Filtration" efforts among others, which show that a "new" air filter will pass far more particulate in the early stages of it's life contrasted to it's maturity efficiency increase. Specifically, approximately 90% of a common air filter's total volume of passed particulate will happen in the first 10% of it's useable life-cycle. Unless one lives in a horribly dusty area, frequent air filter changes are simply not a good idea. An OEMs air FCI (filter change interval) is often VERY conservative, and would have you change filters far, far too often. I have installed an vacuum gauge on my air intakes (car and truck) so that I can monitor the WOT effect of the filter's restriction. While my manual would call for 30k mile intervals, the reality is that I could EASILY go 2x that factor, given the approximate loading curve it seems to exhibit, and quite possibly more.

It is true to say that air filtration is very important to reduce the contamination in the lube sump and reduce wear rates for the engine. But frequent air filter changes are the exact opposite of what one should be doing. It's counter-intuitive to the typical anal-retentive BITOGer approach, but data shows longer air filter changes are the approach which can reduce wear because you're not inducing the very high wear rates associated with the "new" filter being so porous early in its life.
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