Pick the wrong oil and a log splitter crawls on a frosty morning. The ISO 32 vs 46 vs 68 hydraulic oil question comes down to one thing: how thick the oil is. ISO 32 is the thinnest of the three, and ISO 68 is the thickest. The right one is the grade your equipment maker lists in the manual.
This guide uses technical pages from hydraulic shops, equipment makers and Canadian safety groups. You’ll get what the number means, when each grade fits, and how BC weather plays in.
What’s New in Hydraulic Oil for 2026
In September 2026, a US construction equipment magazine looked at winter fluids for heavy machines. It says the industry’s answer to cold starts has more and more been multi-viscosity and high-VI hydraulic fluids. One expert in the piece says these fluids can cut warm-up time.
That same month, a UK hydraulic company updated its oil guide. It asks if ISO 32 is for winter and ISO 68 is for hot weather. Its answer is short: not as a universal rule. The maker’s spec still comes first.
A December 2025 guide from a US farm equipment dealer warns compact tractor owners. Plain hydraulic oil, such as AW46, lacks the friction modifiers that wet brakes need. The dealer says it will cause brake chatter.
What the ISO Number on Hydraulic Oil Means
ISO VG is short for viscosity grade, which is how thick the oil is. A UK hydraulic company says the number is the oil’s nominal viscosity in centistokes, or cSt, at 40°C. So ISO 32 is thinner than ISO 46, and ISO 68 is thicker. The highest or lowest number is not automatically better, the company adds.
Temperature moves the number a lot. A Canadian attachment guide says an AW46 oil is around 46 cSt at 40°C. At 0°C, it’s about 300 to 400 cSt, or six to eight times thicker. At -20°C, it can be 2,000 cSt or more.
ISO 32 vs 46 vs 68 Hydraulic Oil: When Each Grade Fits
As long as your manual doesn’t say otherwise, the sources give the same rough pattern:
Parts counter Three grades
- ISO 32: The thinnest. A UK hydraulic company lists it as best for cold climates and winter use.
- ISO 46: The middle grade. The same company calls it a balanced option for moderate conditions, and commonly used for general work.
- ISO 68: The thickest. The company says it suits hot settings and heavy-duty systems.
- Pump type and load: A Canadian supply site says the right grade depends on operating temperature, pump type and load.
The manufacturer’s manual decides the grade. The UK company’s checklist starts there, with the manual or the manufacturer’s data. If you’ve lost it, check the reservoir label or past service records.
Too thick and too thin both cause trouble. Oil that’s too thick can mean poor starts in the cold and a risk of cavitation, the UK company says. Oil that’s too thin weakens lubrication, so parts wear faster.
AW, HVI and Other Types of Hydraulic Oil
The ISO number tells you thickness. The letters on the label tell you more:
Parts counter Types of oil
- AW (anti-wear): A US farm store says AW fluids also include anti-wear additives.
- HV or HVI (high viscosity index): Viscosity index shows how strongly an oil’s thickness shifts with temperature. A Canadian guide says a 46HV oil is thicker than an AW32 in summer, but flows better than an AW46 in winter.
- Multi-viscosity: A US equipment magazine calls these fluids, along with high-VI oils, the industry’s answer to cold starts.
- Synthetic: An expert in the same magazine says synthetics take longer to thicken in the cold.
Don’t switch oil types on a guess. The Canadian guide says not to mix hydraulic fluid types without flushing the system first. Compatibility with seals matters too. A Canadian supply site says some hydraulic oils can damage seals and hoses, which leads to leaks.
Hydraulic Oil vs Universal Tractor Fluid
Many tractors don’t use plain hydraulic oil at all. On a compact tractor, the transmission, hydraulics, wet brakes and power steering all share the same fluid, a US farm equipment dealer says. That calls for a universal tractor fluid, or UTF.
A US farm store says modern tractors, especially those built after 1980, typically use trans-hydraulic fluids. Older tractors with separate systems may need a universal tractor hydraulic fluid or an ISO-specific hydraulic oil.
Cold Weather Hydraulic Oil in BC
BC isn’t one place in January. A machine on the coast and one working inland can see very different winter mornings, so plan for the coldest temperature where yours works.
In the cold, thick oil struggles to flow into the pump inlet, a Canadian attachment guide says, and the pump partially starves. A US equipment magazine says to match hydraulic oils to OEM specs and the coldest temperatures your machines will see. Many operators in Prairie Canada switch from AW46 to AW32, the Canadian guide says. Only do that if your manual lists the thinner grade.
Warm-up matters as much as the grade. The Canadian guide says most attachment damage in Canadian winters happens in the first 20 minutes. Between 0°C and -10°C, it suggests 5–10 minutes at idle. Seals also get stiffer in the frost, and hose jackets turn brittle.
Signs Your Hydraulic Oil Is Contaminated or the Wrong Grade
Each of these is a reason to have the oil and the system checked:
- Slow or sluggish functions: A UK hydraulic company lists slow or unresponsive hydraulics as a sign of the wrong oil.
- Pump noise: Whining is a symptom of cavitation at start-up, a Canadian guide says.
- Milky or foamy oil: The UK company lists a cloudy or milky look, plus foaming, as reasons to look closer.
- Overheating or clogged filters: Repeated filter blockage and unexpected overheating round it out. Find the cause before you pick a new grade.
Water can cause corrosion and damage to hydraulic parts, a Canadian supply site says.
Leaks need real care, because hydraulic systems hold pressure. A Canadian equipment maker says a leak can release fluid in an almost invisible stream at over 3,000 psi. Injection injuries can happen at as little as 100 psi, and the wound may look like an insect bite. Never use your hand to search for a leak, even in a heavy glove, the maker warns.
In one Nova Scotia case, an unnoticed pinhole leak punctured a mechanic’s glove and the skin of his finger. Lines can hold pressure for weeks or even months with the engine off. A forestry safety group says to shut down and drain the pressure before anyone inspects hoses. Follow the operator’s manual for that step, or leave it to a trained mechanic. If fluid gets under the skin, the maker says to treat it as an immediate surgical emergency.
Topping Up and Replacing Hydraulic Oil Without Mixing Grades
The UK company says there is no universal 2,000-hour, 3,000-hour or annual rule for every hydraulic system. Your manual’s schedule comes first, and oil analysis helps. For tractors on UTF, a US farm equipment dealer says most makers call for 500 to 1000 hours or 2 years, whichever comes first.
When you top up or refill, a few habits help:
Service desk Topping up
- Top up with the same oil: Match the grade and type on the reservoir label or in the service records.
- Small mixes only, and only for a while: The UK company says mixing ISO 32 and ISO 46 is fine only in small amounts for temporary use.
- Big mistake, full drain: If a large amount of the wrong oil went in, a full drain and refill is usually the safest option.
FAQ
What is the difference between ISO 32, 46 and 68 hydraulic oil?
The number is the oil’s thickness in cSt at 40°C. ISO 32 is the thinnest and flows best in the cold. ISO 46 is the middle grade for moderate conditions. ISO 68 is the thickest and suits heat and heavy loads. None is better in general. Your equipment maker’s manual decides which one you need.
Can you mix ISO 32 and ISO 46 hydraulic oil?
Only in small amounts for temporary use, a UK hydraulic company says. Mixing gives a blend that may not match the system’s spec, and the additives may not work as intended. If a large amount of the wrong grade went in, a full drain and refill is usually the safest option. Then go back to one grade.
What is the best hydraulic oil for cold weather?
A thinner oil, such as ISO 32, flows better in the cold. A lift maker says owners with unheated garages may want AW-32. High-VI or multi-viscosity oils also help with cold starts, a US equipment magazine says. Always check that your manual allows the grade, and give the machine a slow warm-up first.
Can I use the same hydraulic oil all year?
Often, yes. A UK hydraulic company says ISO 46 may work year-round in moderate climates, but extreme temperatures may need a seasonal switch. A Canadian guide says a 46HV oil flows better than an AW46 in winter. Check your manual before you plan a switch.
Is AW32 or AW46 better for a car lift?
Neither is better in quality. A lift maker says the numbers refer only to the viscosity, not to the quality. It says owners with unheated garages may want AW-32, while lifts in heated garages can use either AW-32 or AW-46. It doesn’t recommend transmission fluid. Check your lift’s manual for the grade first.
Verdict on Choosing ISO 32, 46 or 68 Hydraulic Oil
The ISO 32 vs 46 vs 68 hydraulic oil choice is about thickness, not quality. ISO 32 flows best in the cold. ISO 46 sits in the middle, and ISO 68 holds up to heat and heavy loads. The equipment maker’s manual has the final say.
Before winter, check the grade your manual lists for cold starts, and give each machine a slow warm-up. If you spot a leak, shut the machine down and keep your hands clear.
Vancouver Auto Parts is a parts store and a full-service repair shop, with shops in Surrey, Vancouver and Port Moody. Our Surrey and Port Moody shops carry industrial parts and chemicals, including hydraulic oils. Bring the grade your equipment maker calls for, and we’ll find the right oil.