Lubrication Consulting / 5 min
What to Consider When Choosing a Hydraulic Oil
Choosing a hydraulic oil isn't just about the ISO VG value. For system efficiency, energy savings and equipment protection, operating conditions, temperature fluctuations and filtration level must be evaluated together. The wrong oil choice leads to system slowdowns, overheating and costly pump failures.
Published by: Rotek Technical Content Team
Content is published based on manufacturer technical documentation and equipment requirements as the basis for final selection. This is general guidance and does not replace equipment-specific manufacturer approval.
Review our editorial guidelinesTable of Contents
- - Pump Types and Tolerances
- - Operating Pressure and Loads
- - Oxidation Life (Thermal Stability)
- - Filtration and Cleanliness Standards (NAS/ISO)
Pump Types and Tolerances
Piston, vane and gear hydraulic pumps have different viscosity and anti-wear (AW) additive requirements. For example, vane pumps need a thinner film layer and strong AW additives at high speeds, while piston pumps require a more stable viscosity index (VI) to prevent leakage.
Operating Pressure and Loads
When system pressure exceeds 250-300 bar, the film layer of standard hydraulic oils can tear. In such high-pressure applications (for example injection molding machines and presses), HLP or HVLP grade hydraulic oils with extra anti-wear (zinc-based or zinc-free) additives should be preferred.
Oxidation Life (Thermal Stability)
Hydraulic oil heats up continuously under pressure. Every 10-degree rise above 60°C cuts the oil's oxidation life in half. In systems that run hot continuously, products formulated with high-quality base oils that offer high thermal stability and do not form sludge should be used.
Filtration and Cleanliness Standards (NAS/ISO)
In modern hydraulic systems, servo valves and proportional valves operate with very tight tolerances. The oil not only needs the right viscosity — it must also have high filterability and meet system cleanliness standards (e.g. ISO 4406 17/15/12).
Frequently Asked Questions
What's the difference between zinc and zinc-free hydraulic oil?
Zinc-based oils provide standard anti-wear protection, while zinc-free (ashless) oils are preferred to avoid damaging silver-plated or special yellow-metal valves, and to avoid problems in wastewater treatment plants.
What is the difference between HLP and HVLP hydraulic oil?
HLP refers to standard industrial hydraulic oils. HVLP describes hydraulic oils with a high viscosity index (less affected by temperature changes) — ideal for construction equipment and mobile hydraulics operating outdoors.
How does hydraulic oil respond when the system overheats?
When a hydraulic system overheats, the oil thins (its viscosity drops). Excessive thinning increases internal leakage, lowers pressure and accelerates pump wear.
What happens if water gets into hydraulic oil?
Water mixing into the oil creates an emulsion. This breaks down the lubricating film, causes cavitation (bubble collapse) and accelerates corrosion in the system.