Hydraulic system oil types and manufacturers' recommendations

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I recently came across the following story posted on a message board by a heavy equipment mechanic:

" I have a customer who recently bought a new service and operator's manual for his old Komatsu PC150 hydraulic excavator. He went to the lubrication guide for the hydraulic system and chose what the chart said was a 10W-40 engine oil and got what Texaco showed as being Komatsu's recommendation. Here is where the problems begin. First you never want to use a detergent oil in a hydraulic system. It can cause anything from foaming in the oil which will cause cavitation in the hydraulic pump and result in premature pump failure, to it picking up waste/contaminants in the oil and depositing it as sludge throughout the hydraulic system.

But according to Texaco EVERY multi-grade oil available in the US is gonna be a detergent oil. Second, and I actually called Komatsu America on this, they DO NOT recommend a detergent oil in the hydraulic system. What is supposed to be in the hydraulic system is either a 10W or a 30W non detergent oil. This tells me that there are a lot of people out there that are blissfully going by the book and slowly killing their machine's hydraulic system. Hopefully at least some of them have asked before they wasted a lot of money and filled their hydraulic system with the wrong oil. My customer is a long-time hydraulic equipment owner, so didn't ask me before he changed his hydraulic oil. The book has to be right, right? Now he's stuck with a 50 gallon system that he can only change 30 gallons of at a time so it's going to cost him big to fill and flush enough times to get all of the detergent oil out of the hydraulic system."

When the OEM doesn't say what they mean, it can certainly make it difficult to follow their oil recommendations. But is a detergent oil, such as a multi-grade diesel engine oil, really detrimental to a modern hydraulic system?

Detergent additives

DIN 51524; HLPD fluids are a class of hydraulic fluids which contain detersive and dispersive additives. The use of these fluids is approved by most major hydraulic component manufacturers and can be advantageous in many applications, including mobile, to prevent build-up of sludge and varnish deposits, which can lead to valve stiction and other reliability problems. The main caution with these fluids is that they have excellent water emulsifying ability, which means that if present, water is not separated out of the oil. Emulsified water reduces lubricity and filterability, can cause corrosion and cavitation, and reduces the life of the oil. These problems can be avoided by maintaining water content below 0.1% - which is not a low water content target for any high-performance hydraulic system. A hydraulic oil that has the ability to emulsify small amounts of water can be beneficial in mobile applications.

Modern oil chemistry means that detersive and air-separation/anti-foaming properties aren't necessarily mutually exclusive. At least one major, mobile equipment manufacturer specifies heavy-duty diesel engine oil to API classifications CD, CF-4, CG-4 or CH-4 as an acceptable alternative to a single-grade, anti-wear, hydraulic oil in their equipment.

Viscosity index improvers

This is not to say that mutli-grade engine oils are suitable for every hydraulic system. Multi-grade engine oil, automatic transmission fluid and anti-wear, high VI (AWH) hydraulic fluid are commonly used in hydraulic systems that experience a wide operating temperature range. These fluids have a higher Viscosity Index (VI) than standard anti-wear hydraulic fluids due to the addition of VI improvers. The higher the VI an oil has, the smaller the variation in viscosity as temperature changes.

In simple terms, this means that if you are running a 10W-40 engine oil in your excavator hydraulics, you can operate the hydraulics with a higher fluid temperature before viscosity falls below optimum, than you could if you were running a single-grade, anti-wear, hydraulic oil.

When selecting a high VI fluid, it is wise to increase the hydraulic component manufacturer's minimum permissible viscosity value by 30% to compensate for possible loss of viscosity as a result of VI improver sheardown (unless the VI improvers used are 'shear stable'). VI improvers can also have a negative effect on the air-separation properties of the oil and for this reason some hydraulic component manufacturers recommend that high VI fluids only be used when operating conditions demand.


As far as hydraulic oil recommendations go, for commercial reasons relating to warranty etc, it is always advisable to follow the machine manufacturer's recommendation. But on the other hand, if you were to realize retrospectively that the hydraulic system of your excavator or dozer had been inadvertently charged with a multi-grade, diesel-engine oil, it's not necessary to press the panic button either. Discussing the application with a technical specialist from the oil manufacturer, in conjunction with the OEM, would however be advisable.

Related articles:

How to select the correct hydraulic oil
Which hydraulic oil should I use?
Why you should keep track of your hydraulic oil top-offs

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