Raul and Buber have the right approach in trying to remove the assumptions from the facts. Along those lines I offer this, rather lengthy
, analysis:
What does oil do?
1. Lubricates: Two types of forces act on oil, compression, such as in the crankcase, and shearing, such as in the gear box and clutch. The heavier the oil weight the more compression it can stand. The “slipperier” the oil the more shear force it can stand.
2. Transfers heat: A measure called the heat coefficient can be determined for any material including oil. In general, organic or carbon based materials have smaller coefficients (transfer heat more poorly) than metallic and non-organic based material. Oil is a carbon based material.
3. Suspends and emulsifies: Ability to keep particles in suspension is related to viscosity and surface tension. Oil has low surface tension but has high viscosity. To improve suspension characteristics, oils may contain detergents. To increase viscosity oils may contain other additives. Oil may also suspend non-particulate liquids, that do not easily dissolve into oil, such as water.
4. Dissolves chemicals: Oil is a poor solvent. However, some chemicals produced inside an engine can dissolve into the oil. Petroleum based chemicals such as gasoline, partially burned gasoline, rubber, plastics and many petroleum based solvents and other chemicals are at least partially soluble in oil.
Do these characteristics change as oil gets used in a SOHC4?
Lubrication: Almost no amount of compression would be sufficient to break down an oil’s lubricating properties. Hence, compression wear has almost no effect on this characteristic. Carbon chains can be “sheared” or “cracked” by high temperature and physical shear force. Hence, oil can loose its slipperiness in this process. However, the point to remember here is that oil is soooooo slippery, that an engine in many lifetimes could not change this factor sufficiently to make a difference. It would overheat and seize first. Even the combination of compression plus heat is insufficient to deteriorate oil to any significant extent in a SOHC4. Topping up would certainly add more lubrication than was lost due to shear or compression.
Transfers heat: This factor actually improves with use. As the content in suspension contains no metallic compounds at first, but may slowly accumulate metal oxides as microscopic dust and or shavings, the oil’s heat coefficient may increase. However, this increase is as minor as almost not worth mentioning.
Suspends and emulsifies: Oil works by forming a thin film between two surfaces that would otherwise make direct contact. The film extensively prevents friction and hence extensively reduces heat from friction. Also, oil prevents metal oxidation to some extend by “bonding” with the metal and preventing oxygen from making those bonds. This applies to all parts of the engine, from rings, to camshaft, to gears and clutch plates. However, there is still some surface to surface contact. Under certain conditions the oil film is scraped away or flung away, and the result is significant engine wear. In some parts of the engine, the surface material is subject to deterioration (burning mostly, such as the clutch). The product of this wear or deterioration is transferred to the oil where it is suspended through agitation and carried away. Oil filtration traps larger particles. But, the microscopic ones remain in suspension as long as the oil is being agitated. Oil that has emulsifier additives (detergents) tends to keep those particles in suspension even when the oil is not being agitated. However, microscopic metal oxides do not always pose a problem, especially if they are smaller than the thickness of the oil film. Particles that small are the bulk of the “black” colour in used oil. Alternately, the use of non-detergent, single weight, super-slippery (synthetic) would also help obviate this condition, as non-microscopic particles would leave suspension when the engine is stopped. “Grit” is the clumping of particles and gums that form as the oil cools and herein lies the main notion that oil must be changed because its “dirty”. Note that changing the filter has an unexpected side effect. New filters are not as efficient at removing smaller particles as are older filters. Of course clogged filters are not at all efficinet.
Dissolves chemicals: The types of chemicals that dissolve in oil may have a deteriorating effect on oil. The effect of diluting the oil with solvents (gasoline, carb cleaner, tetra-chlorides, sulfates, etc), is to alter its viscosity and/or slipperiness. This is certainly one reason to change oil. However, under normal conditions, these chemicals are not present insufficient quantities to alter the oils characteristics in a significant way. But, if the oil smells of gasoline, or rotten eggs, or strawberries, its probably sufficiently contaminated to make a change.
