Predictive Maintenance Services, Inc.



Acid Number

Base Number

Particle Count


Fuel Dilution





Viscosity is one of the most important indicators of lubricant health but is too often disregarded as a condemning parameter worth taking seriously.

Consequences of high viscosity
Foaming, increased energy consumption, lubricant starvation due to lack of flow (this is compounded at low temperatures – especially at start up), cavitation, and filter by-pass.

Causes of high viscosity
The most common cause of high viscosity is the wrong oil or make-up oil added to the system.  Other causes include oxidation, nitration, soot, water, or contamination with an incompatible fluid

Corrective Action
If the high viscosity is caused by soot or water, they can sometimes be filtered out.  If the correct fluid but the wrong viscosity grade was added to a system, it is sometimes possible do remove some of the oil and add a lower viscosity grade of the same product to blend to the correct viscosity. (This works best with non-engine oils and straight grade oils.)  More often than not, however, an oil change is required.

Consequences of low viscosity
Increased wear due to lack of hydrodynamic lubrication, higher temperatures (increased thermal degradation as a result), poor pump performance, and increased oil consumption in engines

Causes of low viscosity
Wrong oil or wrong make-up oil, fuel contamination in engines, shearing of viscosity improver additives or contamination with aromatics such as solvents.

Corrective Action
If the correct fluid but the wrong viscosity grade was added to a system, it is sometimes possible to remove some of the oil and add a high viscosity grade of the same product to blend to the correct viscosity.  In most cases, however, the corrective action indicated is to change the oil.

Alarm levels
New ISO viscosity grade oils (typically most industrial hydraulic, gear and bearing lubes) such as AW hydraulic oil and ISO grade gear oils are required to be within 10% of their viscosity grade and are typically measured in centistokes (cSt) at 40 degrees Celsius.  Condemning limits for used oils is typically 20%, severe is 30% and critical is 40% above or below their ISO viscosity grade.

SAE oils and gear lubes are also typically measured in centistokes but at 100 degrees Celsius.  Condemning limits are shown in the table shown when clicking the blue Viscosity heading on your report or by clicking here


Acid Number (Formerly TAN)
Simply put the Acid Number test measures the amount of base required to neutralize the acid level in one gram of oil. 

Consequences of high Acid Number
As one would expect, high acid content can lead to corrosive wear in the system.  Yellow metals such as copper, brass and bronze are often most at risk.  Additives in the oil designed to neutralize acids may also be consumed which can cause other issues especially in engine oils since detergents and dispersants are often used as alkaline reserves in these fluids.  When those additives are depleted, soot and other undesirable particulates can agglomerate into larger particles or masses that clog filters causing by-pass or settle out as sludge that changes clearances and can clog oil passages.

Causes of high Acid Number
The by-products of oxidation tend to be acidic causing an increase in Acid Number.  Oxidation occurs when the oil is degraded as it is exposed to oxygen.  The oxidation process is accelerated by high temperature and foaming.  Wear metals and contaminants can also act as catalysts that accelerate the oxidation process.   Another cause of sharp increases in Acid Number can be additions of the wrong make-up oil. In Natural Gas and Sour Gas engines the quality of the fuel can have a significant effect on Acid Number.  Water is another cause of increased Acid Numbers.  Water can combine with other elements or contaminants in the oil to form acids.

Corrective Action
It is sometimes possible to lower the Acid Number by filtering out the acidic by-products in the oil.  The addition of fresh oil or make-up oil “sweetens” the additive package and helps reduce the acid content.  Most often an oil change is indicated.  A much better approach is proper filtration and proactive maintenance that keeps the oil cool, clean and dry which extends the life of the oil by eliminating the causes of and catalysts for acid formation.

Causes of low Acid Number
Low Acid Number is most likely caused by the wrong oil added to the system.

Alarm Levels
Some lubricant manufacturers and/or equipment manufacturers will specify an alarm limit for their oils.  The rule of thumb is an increase of two times the Acid Number of the new oil or an increase of one whole number.  In the case of Natural Gas or Sour Gas engines, when the Acid Number and Base Number are equal an oil change is generally indicated.



Base Number (formerly TBN)
The Base Number is a measurement of the alkaline reserve of oil.  Base Number testing is almost exclusively used for engine oils. 

Consequences of low Base Number
The additives that provide the alkaline reserve in engine oils are primarily detergents, dispersants and corrosion inhibitors.  A low Base Number is an indicator that these additives have been consumed to the point where the other functions they perform can also be affected.  Corrosion, soot loading and clogging of filters, increased viscosity, engine deposits, increased viscosity, sludge deposits and corrosive attack are all possible.

Causes of low Base Number
As the acidic by-products of combustion and oxidation are neutralized by the additives, the alkaline reserve, or Base Number, reduces.  Other forces at work that these additives are designed to combat, such as high soot levels caused by clogged air cleaners, improper air to fuel ratios, faulty injectors or bad fuel, can also deplete additives sooner because they have to work harder at other functions. 

Corrective Action
Typically the appropriate corrective action is to change the oil.

Causes of high Base Number
Typically the wrong make-up oil added to the system.

Corrective Action
Typically the appropriate corrective action is to change the oil.

Alarm Limits
Some engine manufacturers and lubricant manufacturers set their own guidelines for acceptable Base Numbers but the rule of thumb is one half the value of the new oil if OEM or lubricant manufacturing guidelines are not known. In Natural Gas and Sour Gas engines, when the Acid Number and Base Number are equal is the condemning point.  Gasoline engine oils often start with low Base Numbers since they don’t have the same operating conditions as diesel engines; very low or undetectable Base Numbers are not uncommon so it is not always used as a condemning test in gasoline engines. 

It should also probably be noted here that the Base Number of new oils is measured with a different test than used oils – ASTM D-2896 for new oils and D-4739 for used oils.  D-4739 will typically give a result that is 1 to 1 ½ numbers lower for new oils than the appropriate test, D-2896.  We mention this because oil with very low time or miles may seem to have experienced a significant reduction in Base Number from the new oil specification when in fact the difference is due to the test method.

Particle count
Simply put, a particle count is a count of the number of particles in six different size ranges per milliliter of oil. 

Consequences of high particle count
High particle counts indicate the presence of particulate matter in the oil.  The particle counter does not identify what the particles are so they may be soft or hard and wear causing so the result of high particle counts varies.  They include abrasive wear, clogged filters, sticking valves, shortened pump life, catastrophic failure, cutting wear, foaming, fatigue spalling and component failure 

Causes of high particle count
There are basically 4 ways particulate matter enters the oil system. 

Built in – this is dirt, dust, shot peen or sand blast residue, lint, honing grit, weld spatter, cutting and drilling swarf etc. that are delivered with the equipment.

Wear – as surfaces rub, roll or slide against each other, particles are generated and carried away by the oil.

Breathed in – most equipment is equipped with breathers to allow for expansion and contraction of the fluid.  Airborne particles enter the system through unfiltered breathers, missing or faulty air filters on compressors and engines.

Poured in – adding oil from dirty containers or using dirty funnels allows particles to enter the system.  Also, opening a fill plug without first cleaning it and the surrounding area allows dirt and debris to fall into the oil.

Corrective Action
Filtration normally can remove particulate matter effectively from oil.  The micron rating, size and type of media depends on the target cleanliness level of the system.  High viscosity oils can also present filtration challenges.  Sometimes when the sump size is small, changing the oil is the more economical solution.  A good rule of thumb for keeping particulate matter out of oil is to filter it every time it is moved from one container to another.

Alarm Limits
Particle count alarm limits depend on the components in the system.  There is a chart provided with suggested cleanliness levels by clicking the blue Particle Count heading on your report or by clicking here. Remember that systems don’t fail because they are too clean.

Water contamination should be taken very seriously. Water is almost always an indication of outside contamination; however, for water/glycol fire retardant fluids, the water content is essential and the percent is important.  Guidelines here do not pertain to water/glycol fluids.  

Consequences of high water
Water can emulsify in oils causing increased viscosity. Water may also cause poor lubrication, rust and additive drop out. 

Causes of high water
Condensation caused by moist air drawn into the system through the breathers.  Failure of a heat exchanger (also called an oil cooler).  Head gasket failure in engines (antifreeze may also be present in most engines). Pin holes in or failure of piston liner or seals in engines (antifreeze may also be present).  Water contaminated make-up oil.  Fill caps left open or missing (rain or moist air).  

Corrective Action
Change oil.  Water absorbing filtration, vacuum dehydration, centrifuge.

Alarm Limits
Alarm limits are very specific to the type of system.  As a general rule, 0.1% or 1000 ppm condemns most fluids.  Some sensitive systems and fuels have limits as low as 200 ppm.  Some like Morgoil systems may be several percent. 

Fuel Dilution
Fuel dilution is the presence of unburned gasoline or diesel fuel in the crankcase.

Consequences of fuel dilution
Fuel dilution lowers viscosity which reduces the hydrodynamic lubrication properties (oil film separation) of the lubricant.  This oil film normally separates moving parts; however, when fuel dilution occurs, metal to metal contact may occur and accelerate wear rates.

Causes of fuel dilution
Incomplete combustion leaves unburned fuel in the cylinder which over time is drawn by the pistons into the oil sump.  Worn rings accelerate the process by allowing exhaust gases and unburned fuel to enter the oil sump in a process known as blow-by.  Improper air to fuel ratios, poor timing, clogged air filters, faulty injectors and bad fuel are all causes of unburned fuel in the cylinder.

Corrective action
Drain engine oil and change filters. Check for operational causes of fuel dilution such as: excessive idling, low engine temperatures, stop and go traffic, overloading and lugging of the engine. Make sure the timing is correct, fuel pressures are correct, air filters are not clogged – all affect the air to fuel ratio. Check injectors for proper spray pattern and operation. Perform a compression test to check for bad rings. 

Alarm level
Some fuel dilution is inevitable and generally less than 3% is acceptable even though some loss of viscosity is probable even at this level.  3% to 5% is considered abnormal and should trigger an oil and filter change.  Over 5% is generally considered excessive.  Some EGR engines and those that deliver unburned fuel to purge particulate traps in the exhaust system via the cylinders will often allow higher fuel dilution levels.  At levels above 5%, however, the oil is thinned to the point that low viscosity condemns the oil.  Check with your OEM for allowable fuel dilution levels.


Soot is a normal by-product of combustion of hydrocarbon fuels and enters the oil by leaking past the piston rings in a process known as blow-by. 

Consequences of soot
Soot and the wear and contamination that become trapped in it are abrasive and accelerate wear.  Soot also increases viscosity which consumes more energy, and can cause lubricant starvation – especially in colder temperatures. Soot blocks off filters causing them to by-pass and allow unfiltered oil to circulate through the engine. 

Causes of soot
Soot levels increase when the air to fuel ratio becomes rich due to faulty injection, poor fuel, clogged air cleaners, heavy loads, hunting and poor compression.  

Corrective action
The immediate corrective action is to change the oil and filter.  A more proactive approach is to identify the cause of high soot levels and correct it so soot levels are controlled going forward. It should also be noted that about the year 2007, Exhaust Gas Recirculation (EGR) engines became common and soot levels began to rise.  

Alarm levels
EGR engines will contain more soot as a rule than other engines and some engine manufacturers allow more soot than others, but as a general rule, less than 2% is normal. 2-3% is abnormal and 3-4% is severe. Over 4% is critical.

When oil is exposed to air, a reaction occurs known as oxidation.  Heat, wear debris and contamination can accelerate the oxidation process. 

Consequences of oxidation
Most by-products of oxidation are acidic so generally an increase in Acid Number also occurs.  Oils become dark and viscosity increases.  Antioxidant additives are sacrificed to combat the problem and will become depleted. Sludge and other deposits can form. 

Other effects of oxidation residue deposits are sticking valves, plugged oil passages, bearing wear (due to interference with hydrodynamic lubrication), filter plugging and inefficient oil cooler performance. 

Causes of oxidation

High temperatures, water contamination, over-extended oil drains, wear and contamination.  It should also be noted here when changing from one oil brand to another oxidation values can sometimes increase dramatically.  The increase is due to additive chemistry in the current oil displacing the additive chemistry of the previous oil.  The previous oil additives along with any deposits on the system surfaces are carried away by the oil and detected by FTIR as oxidation residue. High values after changing brands of oil does not indicate the previous lubricant was inferior but is a result of different additive chemistry and will normalize after one or two oil changes.


Corrective action
Since oxidation is a chemical reaction, an oil change is generally indicated.  Some extremely fine filtration can remove the oxidation by-products from the oil and actually reduce the Acid Number but it is generally advisable to add some make-up oil to “sweeten” the antioxidant additive.  Also check for the cause of oxidation mentioned above and correct those problems to reduce oxidation levels going forward. 

Alarm levels
Oxidation is detected by FT-IR technology and is based on the baseline numbers of the new oil. 


Similar to oxidation but caused by exposure to nitrogen (about 78% of the atmosphere) 

Consequences of nitration
Nitration by-products are commonly associated with varnish formation. Varnish is also unstable in oil and deposits on component surfaces. Varnish is typically hard and reddish-brown; it deposits either where the oil degradation has occurred or throughout the system. Varnish can cause sticking valves (especially servo valves) and interfere with heat transfer.

Causes of nitration

High temperatures, overextended oil drains and may also be caused by improper air to fuel ratios in Natural, CNG and LP gas engines.

Corrective action
As with oxidation, typically an oil change is indicated.  When severe varnishing has occurred, a drain and flush may be accompanied by a solvent rinse to break-down and suspend the hard varnish deposits so they can be flushed away.

Alarm levels
Nitration is detected by FT-IR technology and is based on the baseline numbers of the new oil.