In hydraulic power-transmission engineering, hydraulic oil is not merely a passive fluid that conveys pressure; it is also a dynamic system element that lubricates moving components, carries system heat back to the tank and supports internal sealing. The oil can perform these critical functions consistently only when its viscosity (its resistance to flow, or thickness) remains within optimum limits. The viscosity of all petroleum-based fluids is directly sensitive to temperature; selecting the correct viscosity grade during seasonal transitions, when ambient temperatures change, is therefore a fundamental rule for system health.

Effects of Temperature on Viscosity and System Components

Hydraulic fluids become thinner as they heat up and thicker as they cool down. Either extreme can create different forms of instability in the hydraulic circuit:

Low Temperatures (Excessive Thickening)

During winter or in extremely cold regions, where temperatures such as -30°C and -40°C are encountered in countries including Russia, hydraulic oil becomes thicker. If the fluid exceeds the maximum permissible start-up viscosity specified in the catalogue (750 mm²/s), the pump has difficulty drawing oil from the tank. The resulting high resistance in the suction line causes gas or vapour bubbles to form and collapse in the oil, leading to cavitation damage and high pump noise.

High Temperatures (Excessive Thinning)

During summer or under high-thermal-load field conditions such as those in the Middle East, hydraulic oil can become excessively thin. If its viscosity falls below the minimum safe limit specified in the catalogue (12 mm²/s), the protective oil film between moving metal surfaces breaks down. This accelerates friction-related wear on pump gears, cylinder stages and sealing elements. Thin oil also leaks back more readily through the pump’s internal clearances, reducing the pump’s volumetric efficiency, or net delivered flow.

Oil Selection Guide According to HMLIFT Pump Catalogue Standards

Although the system’s continuous operating range is defined as 0°C to 80°C, optimum operating efficiency is achieved in the 50-60°C temperature band. Based on the SAE and ISO industrial standards, seasonal grades are balanced as follows:

• Winter / Cool Climates (SAE 10W / ISO 32): Low-viscosity industrial hydraulic oils that retain flowability at low temperatures and minimise cold-start stress should be preferred.
• Standard Summer / Temperate Climates (SAE 20W / ISO 46): This is the standard hydraulic-oil group that provides a balance between film thickness and flowability at normal field temperatures.
• Extremely Hot / Heavy-Duty Conditions (ISO 68 – ISO 86): In regions where ambient temperature exceeds 40°C, these higher-viscosity heavy-duty fluids resist thinning under high heat and preserve sealing and lubrication.

Fluid Replacement Rules During Seasonal Transitions

Oil changes should be based not only on the calendar but also on the region’s day- and night-time temperature trends. Good maintenance practice includes draining all old oil from the tank, replacing the return-filter elements so they are suitable for the new fluid, and filling the system under clean conditions to prevent contamination.

Conclusion

Selecting hydraulic oil to suit seasonal conditions is an engineering requirement that directly affects the field performance and component life of vehicle-mounted equipment. Keeping the fluid’s temperature-viscosity balance within catalogue limits supports high-efficiency operation of both the pump and the entire system.