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here is cut and paste - it is a good read.
SECTION 2 – MOTOR OIL VISCOSITY SELECTION
THE BENEFITS OF USING THINNER OIL:
• Thinner oil flows quicker at cold start-up to begin lubricating critical engine components much more quickly than thicker oil can. Most engine wear takes place during cold start-up before oil flow can reach all the components. So, quicker flowing thinner oil will help reduce start-up engine wear, which is actually reducing wear overall.
• The more free flowing thinner oil at cold start-up, is also much less likely to cause the oil filter bypass to open up, compared to thicker oil. Of course if the bypass opened up, that would allow unfiltered oil to be pumped through the engine. The colder the ambient temperature, and the more rpm used when the engine is cold, the more important this becomes.
• Thinner oil also flows more at normal operating temperatures. And oil FLOW is lubrication, but oil pressure is NOT lubrication. Oil pressure is only a measurement of resistance to flow. Running thicker oil just to up the oil pressure is the wrong thing to do, because that only reduces oil flow/lubrication. Oil pressure in and of itself, is NOT what we are after.
• The more free flowing thinner oil will also drain back to the oil pan quicker than thicker oil. So, thinner oil can help maintain a higher oil level in the oil pan during operation, which keeps the oil pump pickup from possibly sucking air during braking and cornering.
•The old rule of thumb for desired oil pressure, that we should have at least 10 psi for every 1,000 rpm, pertains to, and is highly recommended for High Performance and Racing engines. Engine bearing clearances are primarily what determines the oil viscosity required for any given engine. (NOTE: Viscosity does NOT determine an oil’s wear protection capability, like many people think. Wear protection capability is determined by an oil’s additive package, which contains the extreme pressure anti-wear components. That is why 5W30 oils can perform so much better than thicker oils in my wear protection capability testing). But, whatever the bearing clearance, for High Performance and Racing engines, it is best to run the thinnest oil we can, that will still maintain at least the old rule of thumb oil pressure, even if that means using a high volume oil pump to achieve that. A high volume oil pump/thinner oil combo is much preferred over running a standard volume oil pump/thicker oil combo. Because oil “flow” is our goal for ideal oiling, NOT simply high oil pressure. So, one of the benefits of running a high volume oil pump, is that it will allow us to enjoy all the benefits of running thinner oil, while still maintaining desirable oil pressure.
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But, for normal daily driver street engines, it is acceptable to use the old rule of thumb only as an “approximate” general guideline, not an “absolute requirement”. And for those engines, no matter what their bearing clearance is, it is best to run the thinnest oil we can, that will still maintain at least “reasonable” oil pressure, that is not too far below the old rule of thumb oil pressure.
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Using thicker oil just to achieve higher oil pressure, will simply reduce oil flow for no good reason. The oil pump relief valve determines the max oil pressure an engine can make, no matter what the oil viscosity is. And in some engines, the relief valve limits oil pressure to a max of 65 psi. But, that does not mean the engine’s redline has to be limited to exactly 6,500 rpm because of that. Oil pressure does NOT determine the engine’s redline, the mechanical design of the engine does.
Plain bearings, such as rod and main bearings, are lubricated by oil flow, not by oil pressure. Oil pressure is NOT what keeps these parts separated. Oil pressure serves only to supply the oil to the clearance between the bearings and the crankshaft journals. Those parts are kept apart by the incompressible hydrodynamic liquid oil wedge that is formed as the liquid oil is pulled in by the spinning crankshaft. As long as sufficient oil is supplied by the necessary oil pressure mentioned above, no wear can occur. And the higher flow rate of thinner oil, supplies more oil volume to the main and rod bearings, which also helps ensure that the critical incompressible hydrodynamic liquid oil wedge is always maintained.
Thinner oil will of course flow out from the bearing clearances quicker than thicker oil will. But, by making sure there is sufficient oil pressure as mentioned above, the oil supply will always stay ahead of the oil flowing out, which will maintain that critical incompressible hydrodynamic liquid oil wedge.
• Oil flow is what carries heat away from internal engine components. Those engine components are DIRECTLY oil cooled, but only INdirectly water cooled. And better flowing thinner oil will keep critical engine components cooler because it carries heat away faster than slower flowing thicker oil can. This is especially important with plain main and rod bearings, since the flow of oil through the bearings is what cools them. If you run thicker oil than needed, you will drive up engine component temps.
Here are some comparison numbers from an 830 HP road race engine on the track:
15W50 oil = 80 psi = 265* oil sump temperature
5W20 oil = 65 psi = 240* oil sump temperature
Here you can see how the thicker oil flowed more slowly through the bearings, thus getting hotter, driving up bearing temperatures and increasing sump temperatures. And the thinner oil flowed more freely and quickly through the bearings, thus cooling and lubricating them better than thicker oil, while also reducing sump temperatures.
Here’s some additional background on all this – You might be surprised by how much heat can be generated just from an oil’s internal friction, though friction may not the best term to use here. It is probably better to think of this as the heat generated due to the shearing action taking place within the oil.
It is the shearing action of the oil between the crankshaft and bearings, while the engine is under a heavy loading condition, that generates the bearing heat that we are concerned with. The oil wedge formed as the crankshaft pulls oil in and around the clearance as it spins, is liquid oil. And since liquids cannot be compressed, the oil wedge itself is what carries that heavy engine loading (oil pressure serves only to deliver oil to the crank/bearing interface) and prevents the crankshaft and bearings from coming in contact with each other, once the engine is running. Cold start up after sitting, is when the bearings and cranksaft start out in contact with each other.
The difference in flow rate, and the difference in shearing generated heat, is why the viscosity used, makes a difference in bearing and sump temperatures. Thicker oil which flows more slowly and generates more heat from shearing, it is not carrying heat away and cooling the bearings as well or as quickly as it could, so that drives up bearing temps. This in turn, causes hotter oil to be coming out of the bearings and into the sump, which is why we see higher temps on a gauge. That is the opposite of what we want.
On the other hand, quicker flowing thinner oil, not only generates less heat from shearing, but it also carries heat away much quicker, keeping bearing temps down. And this means the oil coming out from the bearings, and going into the sump, is also cooler. And that is why we see the cooler sump temps. This is precisely what we saw with the road race engine example above.
If an engine is running hot, use a thinner oil to increase flow, increase internal component cooling, and help keep sump temperatures down. Keeping oil temps down is important to help keep oil below the threshold of thermal breakdown.
• Thinner oil will typically increase HP because of less viscous drag and reduced pumping losses, compared to thicker oils. That is why very serious Race efforts will generally use watery thin oils in their engines. But, an exception to this increase in HP would be in high rpm hydraulic lifter pushrod engines, where thinner oil can allow the lifters to malfunction at very high rpm. In everyday street vehicles, where fuel consumption is a consideration, thinner oils will also typically increase fuel economy. The majority of new cars sold in the U.S. now call for 5W20 specifically for increased fuel economy. And now Diesel trucks are increasingly calling for 5W30, also for fuel economy improvement.
• Relatively few engines are built with loose enough bearing clearances, to ever need to run oil thicker than a multi-viscosity 30 weight (though some may need a high volume oil pump). The lower the first number cold viscosity rating, the better the cold flow. For example, 0W30 flows WAY better cold than 20W50. And 0W30 flows WAY better cold than straight 30wt, which is horrible for cold start-up flow and should be avoided at all cost. And the lower the second number hot viscosity rating, the better the hot flow. For example, hot 0W30 flows WAY better hot than 20W50.
