Hydraulic oil transmits power, lubricates pumps and valves, and carries heat and contamination away. Using the wrong type can mean slow, noisy or damaged hydraulics. Equipment makers specify the fluid type and viscosity; this page explains the vocabulary so the spec in your manual makes sense.
Two broad families
- Dedicated hydraulic oils are used in log splitters, presses, lifts, dump bodies and many industrial and construction systems. They are usually labeled by ISO viscosity grade.
- Tractor hydraulic/transmission fluids serve a shared sump that feeds the transmission, hydraulics and often wet brakes. They are formulated for gears, clutches and brakes as well as pumps.
Do not substitute one for the other unless your manual allows it.
What ISO 32, 46 and 68 mean
The ISO viscosity grade is the oil’s approximate viscosity at 40°C (104°F). A lower number is thinner. Many systems specify ISO 32 for colder conditions and ISO 46 or 68 for warmer conditions or higher loads. Some equipment uses a multi-viscosity hydraulic oil to cover a wider temperature range.
Contamination is the big enemy
Dirt and water cause most hydraulic wear. Keep fill caps and containers clean, change filters on schedule, and watch for milky fluid, which can indicate water.
Hydraulic system checklist
- Check the level with cylinders in the position the manual specifies.
- Look at the fluid for milkiness, dark color or a burnt smell.
- Inspect hoses for cracks, chafing and leaks. Never check for leaks with your hand; pressurized fluid can penetrate skin.
- Clean around the fill cap before opening it.
- Change filters at the interval in the manual, and when changing fluid.
- Use the specified fluid type and grade, especially in shared-sump tractors with wet brakes.
- Warm up gently in cold weather before working the hydraulics hard.
Frequently asked questions
Can I use tractor hydraulic fluid in my log splitter?
Some splitter makers allow it, others specify a dedicated hydraulic oil grade. Check the splitter manual.
What happens if hydraulic oil is too thick in the cold?
Slow response, pump noise and possible pump damage from cavitation. That is why many systems call for a lower viscosity or all-weather fluid in winter.
Why is my hydraulic fluid milky?
Usually water contamination, often from condensation or a bad seal. Find the cause and replace the fluid.
AMSOIL products for this job
- AMSOIL ISO 32 Synthetic Anti-Wear Hydraulic Oil. AMSOIL says: “Anti-wear protection for high-pressure systems”; “Stays fluid in extreme cold”.
- AMSOIL ISO 46 Synthetic Anti-Wear Hydraulic Oil. AMSOIL says: “Anti-wear protection for high-pressure systems”; “Ultra-high viscosity index”.
- AMSOIL Synthetic Tractor Hydraulic/Transmission Oil. AMSOIL says: “Excellent hot- and cold-weather protection, reducing the need for seasonal change outs”; “Superior anti-wear additives increase pump and valve life”; “Friction-modified formulation reduces chatter and provides smooth operation”.
Product wording above is quoted or summarized from amsoil.com. Read each product’s label and data sheet for directions.
Use AMSOIL’s official lookup guide to see what is listed for your vehicle or equipment, then order online whenever it suits you.