In addition to considering working pressure, temperature, and operating environment when selecting hydraulic oil varieties, there are also some more detailed factors: such as the type of pump, lubricating components, hydraulic transmission systems, and environmental protection factors.
★ According to pump type

For example, based on working pressure and temperature: pressure < 7.0Mpa, temperature < 50℃, use HL; pressure between 7.0 and 14.0Mpa, temperature between 50 and 70℃ use HL or HM; pressure > 14.0Mpa, temperature > 70℃ use HM.

According to pump type: for vane pumps, it is recommended to use HM with high zinc content (Zn in oil > 0.05%); for plunger pumps, use ashless anti-wear hydraulic oil, generally HL type hydraulic oil can be used under normal pressure; if both plunger pumps and vane pumps are present, use HM with low zinc (Zn in oil < 0.03%); for gear pumps or screw pumps, use HL or HM.

★ According to special components

For hydraulic systems containing silver-plated components, use special anti-silver hydraulic oil; CNC machine tool oil should use HR high viscosity index hydraulic oil when working indoors; some hydraulic systems with high-precision servo valves should use clean hydraulic oil; the tool feed system of machine tools requires oil with good stick-slip properties, use HG hydraulic guide oil.

★ Hydraulic transmission system

PTF-1, also known as automatic transmission fluid (ATF), is used in passenger cars and light truck automatic transmission systems; PTF-2 is used in heavy-duty truck and off-road vehicle transmission systems; PTF-3, also known as hydraulic and transmission dual-use or hydraulic, transmission, and brake triple-use oil, is used in the hydraulic transmission systems of agricultural and construction field machinery.

★ Environmental protection factors

Biodegradable lubricants include: HETG which is plant oil-based; HEPG which is based on polyether; HEES which is based on synthetic ester; HEPR which is poly-alpha-olefin synthetic oil.

There are five situations defined abroad as harsh working conditions. As long as one of them is met, the oil must be upgraded by one grade or the oil change interval must be shortened. These five harsh conditions are:

① Engines that are frequently in a stop-and-go state, which are prone to low-temperature sludge.

② Long-term low-temperature and low-speed driving (engine normal water temperature around 80°C), which is prone to low-temperature deposits.

③ Working for a long time at high temperatures and high speeds, especially during long-distance fully loaded driving.

④ Medium-tonnage tractors that are overloaded for long periods (with trailers).

⑤ Dusty places (such as mines, construction sites, deserts, etc.) and roads with sand, gravel, and dirt.

Modern high-pressure, high-power diesel engines have significantly increased the power of diesel engines by adopting direct injection, turbocharging, and intercooling technologies. The temperature of the combustion chamber and piston has greatly increased, accelerating the oxidation of engine oil and increasing oil consumption. Additionally, to improve emissions, the position of the first ring groove has been raised, resulting in a much higher temperature in the first ring groove, which makes carbon deposits more likely to form. For engines that use electronically controlled injection, the overall condition requirements are more precise. All of these factors have increased the harshness of lubricating oil operations. The CD-grade diesel oil that has been used in our country for a long time is no longer adequate; it is necessary to use diesel oil of CF-4 grade or higher. Diesel oil products of CF-4 grade or higher have excellent thermal stability and oxidation stability, suppressing the oxidation and thickening of the oil; they possess strong cleaning and dispersing functions, keeping the engine highly clean; and they provide excellent lubrication and wear resistance, protecting components and extending engine life, making them particularly suitable for large buses and heavy-duty trucks that operate on highways.

Customers often believe that because they spent more money on higher quality oil, it should last longer. For example, if they buy high-grade lubricating oil at twice the price of a lower grade, they expect it to last twice as long. This idea is incorrect because:

(1) To achieve the longest engine life, customers should change their engine oil according to the oil change intervals recommended by the equipment manufacturer. The engine is much more expensive than the oil, so the engine's lifespan is far more important than the oil change interval.

(2) Low-quality engine oils contain very few additives to combat soot and high temperatures. Oil should be changed according to the manufacturer's recommendations; high-quality oils contain multiple times the amount of additives compared to low-quality oils and provide a "quality guarantee" for the longest engine life.

(3) Extending the oil change interval to compensate for the oil price does not truly benefit the customer's engine life. Because the longer the engine lasts, the more profit the user can derive from it.

Engine oil is a common term for automotive engine oil. In addition to design factors, engine oil plays a crucial role in the normal operation of the car engine.

The normal operation of the car engine requires engine oil to create a film between moving parts, reducing friction resistance and power consumption, minimizing wear on components, and preventing metal surfaces from corroding; the circulating engine oil carries away metal particles shed from friction, preventing further wear; at the same time, the flowing engine oil dissipates heat generated by friction, preventing moving parts from burning out due to excessive temperature; the viscous engine oil can also form a film between the piston rings and the cylinder wall, providing a sealing effect and enhancing cylinder pressure.

As engines continue to innovate, engine oil is also continuously upgraded to meet higher demands, providing more efficient lubrication, thorough cleaning, faster heat dissipation, tighter sealing, thereby saving fuel, extending oil change intervals, and comprehensively protecting the engine more effectively, resulting in a longer engine lifespan.

The internal combustion engine must have sufficient starting torque to overcome the resistance torque at startup. Among the resistance torques, the friction resistance torque has the greatest impact on the starting resistance torque, which increases with the viscosity of the internal combustion engine oil used.

At ambient temperatures, the internal combustion engine must reach a minimum speed to start, known as the critical starting speed. The critical starting speed for gasoline engines is 30-40 r/min; for direct injection diesel engines, it is 100-150 r/min; and for pre-combustion diesel engines, it is 100-150 r/min. In the severe cold of winter, when temperatures are very low, the engine's critical starting speed must increase, mainly because the viscosity of the internal combustion engine oil increases significantly at low temperatures, leading to a greater resistance torque that needs to be overcome. Clearly, during cold starts at low temperatures, the lower the viscosity of the internal combustion engine oil, the higher the engine's starting speed, and the better the engine's cold start performance. Multi-grade oils have much lower viscosity at low temperatures compared to single-grade oils, making them easier to start. Additionally, multi-grade oils have lower low-temperature viscosity than single-grade oils, allowing them to start at lower temperatures. There are many factors that affect the engine's cold start, such as fuel, engine structure, compression ratio, ignition timing, etc., but under normal conditions for the above factors, the starting performance is determined by the low-temperature viscosity of the oil.

Multi-grade oils are blended from lighter fractions of base oils with viscosity index improvers, low-temperature flow improvers, and other functional additives. Compared to single-grade oils, multi-grade oils have better viscosity-temperature characteristics (the so-called viscosity-temperature characteristic is the property of oil viscosity changing with temperature, usually measured by the viscosity index; a higher viscosity index indicates that the viscosity of the oil changes less with temperature). At the same viscosity at 100°C, multi-grade oils exhibit excellent low-temperature fluidity and high-temperature viscosity retention. Especially in the piston assembly - where the cylinder wall temperature can reach around 250°C to 300°C, and the temperature at the crankshaft and bearing can reach around 100°C to 120°C, these harsh lubrication points often operate in boundary lubrication conditions. When the engine oil temperature exceeds 100°C, the viscosity of single-grade lubricants is much lower than that of multi-grade oils, making the thickness and strength of the oil film critical for lubrication protection in these areas.

Multi-grade oils must use VII, and a good VII not only requires strong viscosity increase ability and good shear stability but also good low-temperature performance and thermal oxidation stability. Among these, shear stability is an important performance indicator that directly relates to the stability of the viscosity grade of multi-grade oils. Poor shear stability of VII can lead to the breaking of the main chain of high molecular weight substances in the oil under shear stress, resulting in a decrease in viscosity and an inability to maintain the original viscosity grade, which adversely affects wear and oil consumption.

The shear rate distribution in different parts of the engine is different, with significant temporary viscosity loss at the bearings, piston assembly, and cylinder wall, making these areas prone to wear and scratches.

Furthermore, in the specifications of multi-grade oils, there are two physicochemical indicators: low-temperature viscosity (CCS method) and boundary pumping temperature (MRV), to ensure that the engine oil has good low-temperature performance during use. The low-temperature performance of lubricating oil is mainly measured by low-temperature viscosity and boundary pumping temperature. The low-temperature viscosity of the oil mainly affects the cold start capability of the engine. If the low-temperature viscosity of the lubricating oil is too high, the resistance moment during crankshaft rotation will be too large, which may prevent reaching the specified minimum speed and make it difficult to start. Additionally, due to the high viscosity of the lubricating oil, the oil pump's ability to pump oil decreases, leading to insufficient oil supply and difficulty in forming effective lubrication, which exacerbates wear at the lubrication points. Therefore, regulations should be established for the viscosity limits of lubricating oil at low temperatures. The boundary pumping temperature of the lubricating oil refers to the minimum temperature at which the oil can be continuously and adequately supplied to the engine pump inlet. Since the engine startup process is very short (especially at low temperatures), the lubricating oil in the crankcase does not have enough time to be pumped to each friction surface by the oil pump. To ensure that each friction surface of the engine receives timely and effective lubrication, it is required that the oil pressure of the lubrication system reaches normal levels in a very short time after the engine starts. Since single-grade oils use heavier fractions of base oils and do not contain viscosity index improvers, they do not have the requirements for low-temperature viscosity and boundary pumping temperature in their specifications. Therefore, their high and low-temperature performance is generally inferior to that of multi-grade oils. Typically, at the same viscosity at 100°C, multi-grade oils have lower low-temperature viscosity and higher viscosities at 150°C and 230°C, which is beneficial for the engine's low and high-temperature starts. Additionally, during normal engine operation, due to high shear rates (around 150°C, 10^6/s), the oil experiences temporary viscosity loss (this viscosity loss can recover and is reversible), generally not less than 3.5 mPa.s. If the viscosity is too low, the oil film is easily destroyed, sealing performance is poor, oil consumption increases, and wear occurs. The working temperature difference in various parts of the engine during use is quite large, ranging from ambient temperature to 300°C, making the viscosity-temperature performance of the oil particularly important. Through the above analysis, it can be seen that the viscosity-temperature characteristics of multi-grade oils are significantly superior to those of single-grade oils.

Advantages of using multi-grade oils: 1. Improve the low-temperature performance of the oil; 2. Save fuel and energy; 3. Reduce wear; 4. Can be used year-round according to local environmental temperatures.

(1) Multi-grade oil has good low-temperature starting performance. Multi-grade oil has strict requirements for low-temperature viscosity to ensure that the engine starts smoothly at lower temperatures. Multi-grade oil uses a cold start simulator to determine low-temperature viscosity at high shear rates. If the viscosity at 100°C is the same, multi-grade oil has a lower low-temperature viscosity and higher viscosities at 150°C and 230°C, which is beneficial for the engine's low and high-temperature starting.

(2) Multi-grade oil has excellent thermal starting performance. In practical use, it is often encountered that the engine, after running for a period at high temperature, temporarily stops and then restarts shortly after. Because the lubricating oil will flow away from the cylinder wall at high temperatures, the oil film is destroyed and loses lubrication, resulting in very high mechanical friction. Currently, many foreign engines require thermal starting performance to be more stringent than cold starting performance.

(3) Multi-grade oil can improve engine wear and oil consumption. Due to the above advantages, compared to using single-grade oil with the same viscosity, it can save 2%-3% of fuel. The main reason for fuel savings is that multi-grade oil has good low and high-temperature starting characteristics. When the engine is operating normally, due to the effect of high shear rates, the oil experiences temporary viscosity loss, and the viscosity of the oil becomes slightly lower, reducing friction between fluids. This is similar to using low-viscosity oil but avoids the disadvantages brought by low-viscosity oil.

The engine oil becomes thicker and gelatinous during the circulation lubrication process due to increased viscosity. The main reason is the oxidation reaction of engine oil under metal catalysis, incomplete combustion of fuel, air, and extremely high temperatures.

Among them, the high-temperature, incomplete combustion fuel gas that flows down into the oil pan from the engine is the main factor for the prolonged baking of the engine oil. Since a certain proportion of antioxidant additives is added to the engine oil, it has a certain antioxidant capacity. If external influences exceed its antioxidant capacity, abnormal results will occur.

The appropriate viscosity is the primary condition for establishing an oil film on the friction surface. To prevent wear on the contact surfaces of machine parts, the lubricant must have sufficient viscosity to ensure that an oil film can form between moving parts at various operating temperatures. The ability of the oil film to bear external loads is also related to the viscosity of the lubricant. Viscosity is the basis for classifying engine oil. However, higher viscosity is not always better, as the viscosity of engine oil is closely related to factors such as engine power, wear of moving parts, sealing degree of piston rings, consumption of oil and fuel, cold start performance of the engine, and temperature of the machine parts. If the viscosity of the engine oil is too high, there are several disadvantages.

(1) Difficulty in starting the engine at low temperatures

When the viscosity of the engine lubricant is too high, it flows slowly. Although the oil pressure is high, the amount of lubricant passing through is not much, especially during low-temperature starts when the oil cannot be supplied to the friction surface in time. This can easily lead to temporary dry friction or semi-fluid friction, exacerbating wear on the machine parts.

(2) Reduced effective power of the engine

Due to the high viscosity of the lubricant, the friction resistance between machine parts increases. The friction area of the moving parts in a car engine is quite large, about 1 square meter. When these surfaces are covered with a layer of lubricant, even a slight increase in oil viscosity can significantly increase the loss of friction power when the machine parts are moving quickly. To overcome the increased friction, more fuel is consumed. On the other hand, with high viscosity oil, the resistance to oil churning increases when the crankshaft rotates at high speed, leading to increased loss power within the engine, thereby reducing the effective power of the engine.

(3) Poor cooling effect

With high viscosity oil, the flow is poor and the circulation speed is slow, resulting in a slow rate of heat removal from the friction surfaces, thus leading to poor cooling effects and a tendency for the engine to overheat.

(4) Poor cleaning effect

With high viscosity oil, the circulation speed is slow, resulting in fewer passes through the filter, which cannot timely remove metal shavings, carbon particles, dust, and other impurities from the friction surfaces. Its cleaning effect is poor. Additionally, compared to low viscosity oil, high viscosity oil has larger residual carbon particles and higher acid values, which also affect its performance.