A short guide to hydraulic oil

A hydraulic oil is basically an incompressible fluid that is used to transmit power within hydraulic systems. It has numerous applications in machinery that range from the brakes on your car, to the stabilisation system of a luxury cruise ship.

What is hydraulic oil made of?

Like most modern lubricants, hydraulic oil is more than just mineral oil. Depending on the application, it may be a blend of mineral oils, ethers, silicone, esters and various other chemicals to achieve the desired physical properties.

Likewise, extra additives are typically included to help the hydraulic oil do its job under varying conditions. Anti-wear additives help protect surfaces and lengthen the life of equipment, while anti-oxidant additives help slow down sludge deposits and reduce the frequency of oil changes. Anti-foaming agents are especially essential in hydraulic oils, because foam is compressible, thus limiting the hydraulic process. It also hinders effective lubrication.

In certain environments, anti-rust and anti-freeze additives may be particularly desirable to prevent oxidisation and allow operation in extremely cold conditions, respectively.

What’s the ISO Viscosity Grade (VG)?

The UK mostly uses the ISO system for grading hydraulic oils. The ISO VG determines the viscosity of the oil, which is basically how “thick” it is. A higher viscosity means the oil will be harder to move, but this can be desirable in some heavy-duty applications. Lubricant manufacturers generally make a range of hydraulic oils with different VGs to meet various needs.

ExxonMobil produces seven different oils in its Mobil DTE 20 Series. Mobil DTE 24, for example, has an ISO VG of 32 that would typically be suitable for applications like high-power machine tools, while Mobil DTE 25 has a slightly higher ISO VG of 46 that may be more suitable for a high-pressure industrial plant. In reality, there can be crossover in applications, so it’s always important to refer to the manufacturer’s suggested grade. Also, if you’re wondering why the series numbers don’t match the ISO VG, it’s because they relate to the kinematic viscosity rating at 40°C.

What about circulating oils?

If you’ve looked at lubricants like Mobil DTE Light and Mobil DTE Oil Heavy Medium, you’ll notice they are described as circulating rather than hydraulic oils, yet they are said to be suitable for moderate severity hydraulic pumps.

Does this mean you can use them instead of the DTE 20 series? This is something you should discuss with the equipment manufacturer and your lubricant supplier.