The viscosity of oils are a most important quality, a key factor in oil performance. If the oil thins out, bearing surfaces will touch (collapse), resulting in metal-to-metal contact. This causes catastrophic bearing wear. Without the lubricant to act as a cushion, bearings and camshafts will gall. Cylinder walls also are subjected to high forces, and will suffer from piston and ring wear.
Viscosity is the thickness or "weight" (w) of an oil. It is represented as a number such as "10w-30." Polymeric thickeners (AKA viscosity improvers or VI's) are added so the oil will thicken as it gets hotter. A typical 5w-30 petroleum oil is made of 5-weight base oil. At zero degrees Celsius (32 degrees F.) it behaves like 5-weight oil. Because of VIís, it will behave like a 30-weight oil at 100 degrees C. (212 degrees F.)
Some places in the piston have a normal operating temperature of at least 600 degrees, and turbochargers get even hotter than that. This causes thermal degradation. The problem occurs during high RPM and temperatures. Oils "shear back" to the lower number--just when it is needed to be thicker. A 5-30 turns into a 5-weight oil.
Petroleum-based lubricants contain very large molecules, and especially suffer from thermal degradation when exposed to high engine temperatures. When this happens, they also form varnish deposits, which stick rings to the pistons and plug up turbo oil passages. Once a petroleum based oil reaches 475 degrees, it breaks down, turns into tar and varnish and then forms hard deposits that block the oil flow.
Petroleum oils also lose considerable viscosity at high engine temperatures and RPM. When petroleum oil enters an area of high stress and heavy loading, such as a bearing, the large molecules align themselves creating a path of least resistance. The rest of the petroleum oil follows this path, instead of coating the entire surface. The oil viscosity quickly drops, and the oil begins shearing back to the base number.
Contaminates such as dirt, moisture and sludge will also lower an oilís capability to maintain viscosity.
To prevent sludge formation, a detergent-dispersant additive is used in engine oil. This additive is usually a high--molecular weight nitrogen compound.
When sludge precursors begin to accumulate in the engine, mainly by-products of combustion blowby , the nitrogen encircles the by-products, keeping them from interacting and in suspension until the oil is changed. If they combine, they'll form long chains of molecules that become sludge deposits.
However, heat and thermal cycling depletes the detergents, then the sludge will begin to coat the inside of the engine, plug oil passages and cause catastrophic engine damage. This is an important consideration any time you extend oil drain intervals over recommended time or mileage.
As the engine operates, combustion gasses acidic by-products from the sulfur in the fuel. The acids combine with moisture (every gallon of gasoline burnt produces Ĺ gallon of moisture) which dissolve bearing surfaces.
All modern motor oils contain sodium hydroxides (NaOH) to combat the acid build up in the oil. As the oil becomes contaminated and turns acid, the NaOH gets used up. Preventing acid buildup is one of the primary reasons for changing motor oil. Short trip driving is the worst kind of driving for this problem as moisture is not boiled off during driving cycles.
Because everyone's driving habits are different, everyone's oil gets contaminated at a different rate. And since various oils contain different amounts of NaOH, the only real way to tell if your oil is acid contaminated is by having it analyzed.
The TBN* represents the number of grams of NaOH per kilogram of oil. New oil comes with a TBN number ranging from 5 to 12, depending on the quality of the oil. Store bought oils have a TBN of 6. High quality synthetics have a TBN of as high as 12. When the TBN drops below 2, itís time to change it.
Taken from here:http://www.waynesgarage.com/docs/oil.htm