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The Difference Between Hydraulic Oil and Engine Oil

Release：Pinuo Environmental Protection Technology　Release time：2026/8/18

Hydraulic oil and engine oil have completely different compositions and functions, and must never be mixed or substituted for each other, as mixing can cause severe damage to equipment.

What are the core differences
Different purposes: Hydraulic oil is primarily used in hydraulic systems to transmit power, ensuring precise operation of systems like brakes and steering; engine oil is specifically designed for the engine's internal components, responsible for lubricating high-speed moving parts and dissipating heat.
Large compositional differences:
Hydraulic oil consists of approximately 99% base oil and less than 1% additives, primarily including anti-wear agents, rust inhibitors, anti-foaming agents, and anti-emulsifiers.
The proportion of engine oil additives is higher (5%-30%), containing detergents and dispersants to remove carbon deposits and sludge, which are special components absent in hydraulic oil.
Viscosity and temperature resistance:
The low viscosity of hydraulic oil facilitates smooth flow and efficient power transmission under high pressure.
The higher viscosity of the engine oil allows it to form a stable oil film under the high-temperature and high-pressure conditions of the engine, with a broader temperature resistance range (approximately -30°C to 150°C).
Appearance distinction: New hydraulic oil is mostly a light yellow transparent liquid with a mild odor, while engine oil has a darker color (light yellow to brown) and a stronger scent.
What are the consequences of mixing them
The Hazards of Using Engine Oil as Hydraulic Oil:
Excessive viscosity can clog the filter element, preventing the normal flow of oil pressure.
The absence of defoamers and emulsion breakers can lead to bubble formation, compromising system stability.
The chemical components in the engine oil can corrode the rubber seals of the hydraulic system, leading to oil leakage and equipment failure.
The Hazards of Using Hydraulic Oil as Engine Oil:
Insufficient viscosity causes the oil film to break easily, leading to direct dry friction between engine components.
Without a dispersant, carbon deposits cannot be removed, and the engine will soon turn into a "trash can.".
Insufficient lubrication leads to accelerated component wear and even engine failure.
The consequences of the two being mixed:
Viscosity changes affect pressure control and flow regulation.
The anti-foaming and anti-emulsifying properties deteriorate, leading to bubble accumulation and damage to hydraulic components.
Moisture is difficult to separate, leading to component corrosion.
How to properly select and use
Check the labels: Engine oil is marked with API grades (e.g., SN, SP), while hydraulic oil is marked with ISO viscosity grades (e.g., ISO 32, ISO 46).
Select oil according to the vehicle manual: Change engine oil when necessary, change hydraulic oil when necessary, and avoid mixing them for convenience.
What to do if you accidentally add the wrong thing
Immediately clean the hydraulic oil tank and remove any residual incorrect oil.
Refill the appropriate hydraulic oil to ensure the system operates normally.

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