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The Importance Of Selecting The Right Hydraulic Fluid

Choosing which hydraulic Fluid to use for a customer’s equipment can seem like a daunting task, especially to new salespeople. There is a whole world of choices out there—not just of brands, but also of varying base oils, additive contents, viscosities, and cleanliness levels. Factor in the addition of specialty products and an array of synthetics, and it’s understandable the selection could be confusing. When it comes to hydraulic fluid selection, a little bit of knowledge goes a long way.


The primary starting point for any application should be finding out what the Original Equipment Manufacturer (OEM) recommended. OEMs will list in
an equipment manual the ideal fluid for their equipment. From this, we hope to get the specifications. Specs are the gold standard for starting points in lubricant selection.

There are two basic types of specs: the first is OEM. This can be challenging to figure out, since details are not always widely published. Many lubrication
professionals prefer to nd OEM specs, since selecting the proper fluid is often as simple as matching the OEM specs listed on Product or Technical Data Sheets (PDS or TDS) to the ones listed in the equipment manual.

The other basic type of spec is that of a more general nature. Sometimes these will be listed with basic information, such as to say that one only needs a hydraulic fluid that is a “46 grade”. If these aren’t listed with a regulating body, it is expected that we use the most generally accepted one. In the case above, that would be an International Standards Organization Viscosity Grade (ISO VG) 46. The ISO VG of any reputable hydraulic fluid should be listed on a PDS. Of course, there is often more listed than just the ISO VG, and all those details can influence product selection.

In lieu of a spec to work with, you may be presented with a list of product names. It is important to be able to talk to your customers about this list of products. Usually, they will want only one of those products specifically listed, for fear of voiding any warranty on their equipment. However, the Magnuson–Moss Warranty Act, in effect since 1975, prevents OEMs from requiring any particular brand in order to protect warranty.

In this case, you can match the attributes of the listed products with an equivalent, or even better, product. At Phillips 66, our Technical Services Hotline handles thousands of these product crossover requests every year, especially with older equipment that lists a product which no longer exists, or has changed

This brings us to another important question to ask: “What product is in there now?” If the product matches with the specs from the manual, great. If not, then we need to ask more questions.

  • Why is it not the same type of product?
  • Was there a performance issue with the equipment that warranted a change?
  • Who made that decision and based on what data?
  • Has the equipment been modified from its OEM design?

History teaches us that customers often get changed to a “close enough” product when switching suppliers. If this has occurred more than once, then the product could have strayed significantly from the original. For this reason alone, it is worth investing a little more time to nd out what specs should be met, as opposed to crossing the wrong product to another product that is ultimately wrong as well.


Mentioned earlier is the fact that there are a multitude of other specifications that could be OEM recommended for a piece of equipment. The following are a few examples:


Choosing the proper viscosity is the single most important aspect to selecting a lubricating fluid. If you get nothing else correct, using the right viscosity will greatly reduce the chances of mechanical failure. That is not to say that additive packages aren’t important. Formulation is vital to success, but there is not an additive out there that compensates for a viscosity that is far off from what it should be.

Related to viscosity is viscosity index (VI). Viscosity Index is a unitless measure of the way that a uid’s viscosity changes in relationship to changes in temperature. All oils change viscosity with temperature change. Like the oils in your refrigerator, they will thin down when they heat up and thicken when they cool down. Fluids with a high VI experience less viscosity change due to temperature variation than lower VI products would. For example: the unaltered VI of a hydraulic fluid made with a quality Group II base oil is typically about 95-110 (Group II base oil VI is 80-120). In comparison, a high-quality hydraulic oil that has an improved VI (improved as in, we put additives in it to make the VI better) may have a VI around 160 or higher.


This brings us to another spec to watch out for: biodegradability, which refers to the way that a fluid and its additive contents break down over time. For example: since ZDDP does not break down rapidly, it is avoided for products that are designed to be biodegradable. Keep in mind that biodegradable is not synonymous with “environmentally conscious” or “green.” A product to be considered “environmentally
conscious” could be biodegradable, or it could be a product that is re-refined or recycled but not biodegradable. To state it another way: some products tagged as “environmentally conscious” do contain ZDDP and, thus, are not biodegradable.

There are several different organizations and test methods to determine biodegradability, so one needs to be mindful when comparing product-to-product that the same testing standards are applied to both.

Below is a highly simplified categorization of biodegradable categories per OECD Method 301B:

  • Non-biodegradable—These products break down 20% or less in 28 days.
  • Inherently biodegradable—Will break down between 20% and 60% in 28 days.
  • Readily biodegradable—Break down 60% or more in 28 days (and achieve 60% within 10 days of first hitting 10% biodegradation)
  • Ultimately biodegradable—Achieves 60% or more break down in 28 days (However, the test can be extended for more than 28 days if samples indicate that biodegradation is still progressing) (no 10-day window)

Additionally, there are other tests for environmental concerns other than how quick a product biodegrades. Keep an eye out for requirements for static sheen. This is an indication of how the fluid does or does not leave a rainbow-looking film on water. There are also a variety of lethal concentration (LC) tests that commonly examine specific types of fish or shrimp to determine if the product is safe to use
close to waterways. There are even tests on certain algae for similar reasons.


What most customers just think of as “zinc” is actually zinc dialkyldithiophosphate (ZDDP). ZDDP has been the most popular lubricant antiwear additive for a long time, and for good reason: it works well. However, there are certain scenarios where ZDDP is not ideal.
For example, many OEMs are now recommending non-zinc antiwear hydraulic fluids. Well known manufacturers such as Hitachi, Case, Komatsu and John Deere are doing this for a variety of reasons including compatibility with the metallurgy inside their equipment. Another case for non-zinc fluids is when there are environmental concerns. If equipment is being used in environmentally sensitive areas (such as
proximity to water), then there may be governmental restrictions on using zinc containing products.


Another factor to consider is the investment into cleaner fluids. Numerous studies over the years have shown that running hydraulic fluids with too many particles in them can increase wear and tear on equipment and cost end users significantly in down time and repair work. But how dirty is too dirty and how do we determine if a product is dirty? The answer to the first part is determined primarily by application. Some equipment like diaphragm pumps can handle a fluid with a significant particle content without much problem. Other applications such as servo valves, spool valves or other components with very tight clearances can be quite sensitive to any dirt or build up. One can expect that most of the pumps and valves used in hydraulic systems are going to fall more toward the latter side and benefit from a cleaner
fluid. This doesn’t consider the cost of changing filters and draining sediment from reservoirs of dirty systems.

When people speak of fluid cleanliness, they are almost always referring to particle count. When referring to particle count, the standard is the ISO Solid Contamination Code. This system gives a consistent methodology for reporting the range of particles in a lubricant that are a set size or larger. If a number is reported for 10-micron particles, that will include all particles that are 10 microns in size, 11 microns, 12 microns, etc. As such, when looking at particle counts, the number of larger particles should be smaller than the number listed for smaller

The standard way that manufacturers report cleanliness of new fluids is R4/R6/R14. This is an indication of the number of particles that are 4, 6, and 14 microns or larger as reported on the ISO 4406:1999 chart. There are limitations to this approach. In lieu of actual particle numbers (which can vary), the system calls for counting the number of particles per milliliter (p/ml) and matching up that count with a number in the ISO Code. The benefit of this is that users are presented with a smaller number that is easier to compare. The downside is that the code covers a wide range of actual particle counts. For example: and ISO code of 20 represents 5,000 to 10,000 p/ml. Another limitation, if we continue with the example, is that an ISO code of 19 represents 2,500 to 5,000 p/ml. So, does a 20 have one more particle p/ml than a 19

or does it have 7,500 more? Because we are dealing with ranges, both could be true. Hence, cleanliness ratings should be used as references. In terms of reporting cleanliness on in-service fluids, one should expect more variety. Often, oil analysis labs have options available on the types of particle count reporting they can offer and usually with the true particle count p/ml.


There are some things that the customer should be upfront with needing from the very start. Like checking to see if there are safety-sensitive requirements for their hydraulic fluids. Although there could be any number of safety-sensitive attributes of a hydraulic fluid, the two that come up most often are food use and re resistance. Food grade materials are subject to standards and approvals from several organizations including Canadian Food Inspection Agency (CFIA), U.S. Food & Drug Administration (FDA), and older United States Department of Agriculture (USDA). The current gold standard lies with what was once known as the National Sanitation Foundation but now known as NSF International (NSF). The three levels of NSF food grade approval that we encounter the most are: H1, H2, and H3. Here is the basic difference: H1 can be used for incidental (but not intentional) food contact, H2 can be used in food processing facilities, but only here there is no chance of food contact, and H3 can only contain very specific materials as it is suitable for probable food contact.

The other safety concern that comes up often is fire-resistance. Fire resistance can be achieved formulaically beginning with the proper base oils that are often water-extended or specialty synthetic. There really is no limit to the applications where one might want to use a hydraulic uid that has a good resistance to combusting, but there are some applications where it is mandatory. If human beings are trapped in a relatively limited space and there is a chance that a hydraulic line break could put a hot fluid near an even greater source of heat, then fire-
resistance in paramount.

One such application is that of underground mining. Mining is a highly regulated world that takes safety very seriously. The Mine Safety and Health Administration (MSHA) has rules and regulations pertaining to a wide variety of concerns in mining including re resistant hydraulic fluids. When dealing with mining applications, do not take chances. If there is an MSHA spec listed as a requirement for that mine, make sure the fluid has that full certification. Sea going ships and many manufacturing facilities could also be strong candidates for re resistant hydraulic fluids.

Many of the selection factors referred to in here are applicable to other product types as well. Make it a point to seek out training that will enable you to better serve your customers. At Phillips 66, we offer a robust training curriculum both online and in-person that can give you this advanced level of knowledge when it comes to product selection and applications.