The oil inside a hydraulic jack does three jobs at once. It carries the force from the pump to the ram, it lubricates the pump plunger and the seals, and it lets the check valves close tightly so the jack holds a load. Most complaints distributors hear from workshops (“it won’t lift”, “it only goes up halfway”, “it creeps down”) come back to the oil: the level, air in the circuit, or the wrong fluid.
This guide is written by the team that fills and pressure-tests every JackMaker jack before it leaves the factory. It covers which oil to use, what not to use, and how to check, refill, bleed and change the oil in a bottle jack or a floor jack.
What oil goes in a hydraulic jack?
Use a dedicated hydraulic jack oil or a good anti-wear hydraulic oil:
- Type: mineral hydraulic oil with anti-wear (AW), anti-foam and anti-rust additives. It must be non-detergent.
- Viscosity: ISO VG 32 for most bottle jacks and floor jacks. The number is the viscosity in mm²/s (centistokes) at 40 °C; grade 32 allows 28.8–35.2.
- Hot workshops or continuous heavy use: ISO VG 46, which stays slightly thicker when warm.
- Freezing workshops: a lighter low-temperature hydraulic oil, so the pump does not feel stiff and the jack does not lift sluggishly.
- Seal compatibility: the oil has to suit the nitrile (NBR) and polyurethane seals and the valve seats inside the jack.
If the jack’s manual or rating label names a grade, use that grade.
The same class of oil is sold under different names. For a jack they are interchangeable:
| Name on the bottle | Where you will see it | What it means |
|---|---|---|
| Hydraulic jack oil | Auto-parts and tool retail | Usually ISO VG 32 anti-wear oil in a small bottle |
| AW 32 | North America | Anti-wear hydraulic oil, ISO VG 32 |
| HM 32 | ISO 6743-4 classification | Mineral hydraulic oil with anti-wear additives |
| HLP 32 | DIN 51524-2, Germany and much of Europe | The same class as HM, with defined minimum properties |
Fluids you should not put in a jack
| Fluid | Why it does not belong in a jack |
|---|---|
| Motor oil (SAE 30, 10W-30, 15W-40 …) | Detergent and dispersant additives; much thicker than jack oil (SAE 30 is roughly ISO VG 100), so the jack pumps slowly and foams |
| Brake fluid (DOT 3, 4, 5.1) | Glycol-based: swells and softens nitrile seals and absorbs water, which rusts the cylinder |
| ATF and power steering fluid | Friction modifiers and seal-swell agents made for transmissions and steering racks, not for jack valves |
| Gear oil | Several times thicker than ISO VG 32; in cold weather the jack barely pumps |
| Vegetable oil, water-based fluids, “whatever is on the shelf” | Oxidise and gum up the valves, corrode steel or freeze |
Wrong fluids are behind many “the jack sinks overnight” complaints that turn out not to be manufacturing faults. If the wrong fluid has gone in:
- Drain the jack completely (see “How to change the oil” below).
- Flush it once: add a small amount of jack oil, pump a few strokes with the release valve open, and drain again.
- Refill with the correct oil and bleed the air.
- If brake fluid was used, or the seals now leak, fit a new seal kit.
How to check the oil level
- Lower the jack fully. Open the release valve and push the ram (bottle jack) or the saddle (floor jack) all the way down. The level is only correct with the ram retracted.
- Position the jack. Stand a bottle jack upright; set a floor jack flat on level ground.
- Clean around the filler plug, so dirt does not fall into the reservoir. On a bottle jack the plug is a rubber bung or small screw on the side of the reservoir; on a floor jack it is on the hydraulic unit, sometimes under a cover plate.
- Remove the plug and look. In most jacks the oil should reach the bottom edge of the filler hole. If you cannot see oil, top it up.
Fill by level, not by volume. The amount differs from model to model; the level is what the pump and valves depend on.
How to add oil to a hydraulic jack
- Keep the ram fully lowered and the release valve open.
- Pour the oil in slowly with a funnel or squeeze bottle until it reaches the bottom edge of the filler hole.
- Wipe off any spill and refit the plug firmly.
- Bleed the air (next section), then check the level once more.
Do not overfill. When the ram goes down, its oil returns to the reservoir. If the reservoir is already full, that oil is pushed out of the plug or vent, or the ram will not retract all the way.
How to bleed air from a hydraulic jack
Air gets in when the oil is low, after refilling, and when a jack has been carried or stored on its side. The signs are a spongy handle, a ram that stops short of full height, or a jack that lifts in jerks.
Bottle jack
- Turn the release valve counter-clockwise about one full turn to open it.
- Remove the filler plug so trapped air can escape.
- Pump the handle six to eight full strokes. This pushes the air out of the pump chamber into the reservoir.
- Close the release valve clockwise, hand-tight, and refit the plug.
- Pump the ram to full height with no load, open the valve and push the ram back down. Repeat two or three times, then recheck the oil level.
Floor jack
On most floor jacks the release valve is opened by turning the handle counter-clockwise. With the saddle fully lowered, open the valve, pump the handle six to eight full strokes, close the valve, then raise and lower the empty saddle two or three times.
If the handle is still spongy after three cycles, look for an oil leak or a worn pump seal: our hydraulic jack repair guide goes through the checks in order.
How to change the oil
Change the oil when it looks dark, milky (water has got in) or gritty, when the jack keeps picking up air, and as routine service:
| Use | Check the level | Change the oil |
|---|---|---|
| Professional workshop, tyre shop, fleet | Every 3 months | Once a year |
| Home garage, occasional use | Before a big job | Every 2–3 years |
To drain the jack, lower it fully, remove the filler plug and turn the jack over a container. A few pump strokes with the release valve open push out the oil left in the pump. Refill with fresh oil, bleed the air, and take the old oil to a recycling point.
Storage habits that keep the oil clean
- Store bottle jacks upright, so air does not reach the pump and oil does not seep from the vent.
- Keep the ram and screw extension fully down in storage. A retracted ram is protected from rust and knocks, and the seals stay covered.
- Wipe the ram before lowering it after dusty or wet work, so grit is not drawn past the seal.
- Keep the filler plug seated; an open hole lets in dust and moisture.
What we do at the factory, and what to ask your supplier
Every JackMaker jack is filled with a specified hydraulic oil and pressure-tested before it is packed, so a new jack should not need topping up out of the box. For distributors and private-label brands we:
- print the oil grade and the refilling steps in your manual, in your language;
- supply small bottles of jack oil as an accessory item that ships in the spare space of a jack container;
- stock seal kits for every catalogue model, so your customers can service jacks locally during the 36-month warranty and after it.
Match capacity and height to the job on our hydraulic bottle jacks and hydraulic floor jacks pages, or request a wholesale quote with the oil grade your market expects on the label.