Jack Oil for Bottle Jacks | What Hydraulic Fluid to Use

By Vyscocity Inc. | July 7, 2025

What Oil to Use in a Hydraulic Jack (And Why It Matters)

Hand operating a bottle jack with a metal handle — used for applying hydraulic oil. Vyscocity branding.

Using the right hydraulic oil keeps your bottle jack working safely and smoothly.

Hydraulic jacks are incredibly useful — until they stop lifting. One of the most common causes of jack failure isn't mechanical. Instead, it's often fluid-related. If the jack is low on oil, filled with the wrong type, or never had its fluid changed, you're setting yourself up for frustration or even failure during a lift.

This guide breaks down the best fluids to use, explains the difference between ISO 32 and ISO 46, walks you through safe refill steps, and shows why using motor oil or ATF can wreck your jack.

Why Hydraulic Fluid Matters in a Jack

Unlike a car engine, which uses oil primarily for lubrication, a hydraulic jack relies on fluid to generate lifting pressure. It's the fluid that moves the ram. Because of this, the wrong viscosity or a contaminated supply can make the jack fail.

Using the wrong fluid can:

Damage internal seals
Cause jerky or slow movement
Allow internal rust
Prevent the jack from lifting or holding weight

For most portable jacks, ISO 32 hydraulic oil is the gold standard. Even so, it's always best to check your owner's manual for specifics.

What Kind of Hydraulic Fluid Goes in a Jack?

That depends on your environment and equipment. Here's a quick breakdown of common options:

Recommended Fluids for Bottle and Floor Jacks

ISO 32 Hydraulic Oil

Best for most portable jacks. It's fast-flowing and performs well in cold conditions.

ISO 46 Hydraulic Oil

Designed for warmer climates and heavier-duty jacks. The thicker profile holds up under heat.

ATF (Dexron/Mercon)

For emergencies only. While it can work short-term, it may swell seals over time.

Motor Oil (5W-30)

❌ Never use it. It's too thick, foams easily, and can damage seals quickly.

✅ Top Pick: AMSOIL Synthetic Hydraulic Oil ISO 32

🧠 Heads-up: AMSOIL ISO 32 and 46 are currently only available in 5-gallon (18.9 L) pails, which may be overkill if you just need a few ounces for a small bottle jack. However, it's a smart buy for those with multiple jacks or shop tools.

ISO 32 vs ISO 46 Hydraulic Oil | Jack Fluid Comparison

Property	ISO 32	ISO 46
Viscosity (cSt @ 40°C)	~32	~46
Cold Weather Use	✅ Better	⚠️ Thicker
Common Applications	Jacks, light hydraulics	Larger lifts, shop equipment
AMSOIL Product	AW Series 32	AW Series 46

ISO 32 flows better in cold weather, making it ideal for bottle jacks in unheated garages or outdoors. ISO 46 is thicker and better suited for warm environments or larger equipment. Always match the viscosity to your jack’s requirements and operating temperature.

ISO 32 vs ISO 46: Which Is Better for Hydraulic Jacks?

ISO 32 is thinner and flows faster. As a result, it works better in cold environments or general-purpose jacks.

On the other hand, ISO 46 is thicker and more stable in hot, high-load conditions. Because of its higher viscosity, it's preferred for industrial or high-heat use.

If you're unsure, go with ISO 32. It's compatible with most portable jacks and lifts.

How to Refill a Bottle Jack with Hydraulic Fluid

Here's a step-by-step guide:

First, lower the jack completely.
Place it upright on a stable, level surface.
Locate the fill plug — usually on the side near the cylinder.
Remove the plug using a flat screwdriver or Allen key.
Slowly add ISO 32 fluid until it slightly overflows from the port.
Next, replace the plug and clean up any excess.
Finally, pump the jack 10–15 times to purge air from the system.

📸 Pro Tip: Use a small funnel or squeeze bottle for better control and zero mess.

How Often Should You Replace Your Hydraulic Jack Oil?

There's no set mileage like with engines. However, here's what we recommend:

Top off yearly, especially for frequently used jacks.
Replace completely if:
- The fluid looks milky or cloudy (moisture contamination)
- The jack hasn't been used in over a year
- You're restoring or servicing older equipment

By staying on top of maintenance, you'll prevent corrosion and extend the life of your jack.

What Happens If You Use the Wrong Fluid?

Choosing the wrong fluid — or mixing incompatible ones — can cause serious issues. For example:

Foaming, which reduces pressure and performance
Seal swelling, cracking, or total failure
Rust from poor fluid chemistry
Inconsistent or jerky lift action

To avoid all of this, always use manufacturer-recommended fluids. One proper refill costs less than a cheap jack replacement.

Troubleshooting Common Hydraulic Jack Issues

Jack won't lift:

Usually caused by low fluid or air in the system. Top off and cycle the handle.

Jack lifts then sinks:

Worn seals or aerated fluid. Replace the fluid and inspect for leaks.

Jerky motion:

Old or contaminated oil. Drain and switch to fresh ISO 32.

Leaking fluid:

Likely bad seals. A rebuild may be required.

Testimonials | Real Users on AMSOIL Jack Hydraulic Oil

My floor jack wouldn't hold pressure anymore. I drained the old fluid and switched to AMSOIL ISO 32 — now it lifts smoother and hasn't leaked since. Totally worth it.

Canadian winters were killing my jack performance. Swapping to AMSOIL's synthetic hydraulic oil made a massive difference.