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Common test items for oil analysis: Viscosity: Viscosity is a measure of a fluid’s viscosity, representing the resistance to flow caused by internal friction within the fluid. High viscosity indicates high internal friction; the greater the molecular weight, the more carbon-hydrogen bonds there are, and thus the greater this force becomes. Viscosity is decisive for various lubricants in terms of quality identification and application determination, as well as for the combustion performance and efficiency of various fuel oils. At the same distillation temperature, petroleum products with alkanes as their main components have low viscosity and good viscosity-temperature behavior, that is, a high viscosity index; in other words, the change in viscosity due to temperature variations is small ； Oils with a higher content of naphthenic (or aromatic) components have higher viscosity, that is, they exhibit poorer viscosity-temperature behavior ； Oils with high levels of gums and aromatics have high viscosity*, the worst viscosity-temperature behavior, that is, the lowest viscosity index. Viscosity is commonly expressed as dynamic viscosity, with the unit of mm2/s. Heavy fuel oil has high viscosity; preheating it to a dynamic viscosity of 18–20 mm2/s (at 40°C) facilitates uniform fuel injection by the nozzles. Char: Char is the dark brown residue formed when the test oil is heated under specified conditions in the crucible of a char determination apparatus, causing the oil to evaporate and decompose. The electric furnace method specifies that the furnace temperature be maintained at 520±5°C to evaporate and decompose the residues. Generally, for diesel residue specifications, the sample oil is distilled until only 10% remains before evaporation and decomposition occur. The 10% distillation residue carbon is referred to as carbon; this value for the 10% distillation residue carbon is much higher than that obtained from the total combustion carbon test, whereas heavy fuel oils are required to be tested for total carbon. Large low-speed diesel engines can use heavy fuel oil containing 10% carbon residue. The char value affects coking and carbon deposition in the combustion chamber. However, the wear of the cylinder and piston depends not only on the amount of carbon residue but also primarily on the hardness of the carbon. Carbon deposits with a high sulfur content are hard and tough, resulting in greater wear. Boiling range: The boiling range refers to the temperature range that indicates the evaporation characteristics of a petroleum product, as determined by its distillation under specified conditions, from the initial boiling point to the final boiling point. It is primarily used to determine the proportion of light and heavy fractions in the oil, as well as to control product quality and performance. The boiling range is an important indicator that ensures rapid evaporation, vaporization, and combustion of diesel in the engine’s combustion chamber. Light diesel full distillation range 160~365℃ ； Heavy diesel is used in low-speed diesel engines, where there is sufficient time for atomization and evaporation; strict requirements are not placed on its boiling range, which is generally between 250–450°C. Currently, mixed fuel oils are being used in medium and low-speed large and medium-sized diesel engines