Condition-Based vs. Interval-Based Oil Changes

Noria Corporation
Tags: oil analysis, oil changes

Condition-Based vs. Interval-Based Oil Changes

Should you use oil analysis to determine when to change your oil, or does an interval approach work best? Every machine is different.

There are many factors that influence the decision of when to change oil. Because the decision to schedule oil changes must take into consideration a number of factors, the tool below was developed to guide you. It is a systematic approach to making a maintenance and reliability optimization decision with respect to interval-based vs. condition-based oil changes. Be sure to also consider whether you have set appropriate cleanliness targets to achieve your reliability goals. Give it a try!

Condition-Based Oil Change Wizard

Volume of fluid contained in machine compartment (including circulating lines)

Makeup fluid as a % of total volume added per year or between oil changes

Cost of lubricant or hydraulic fluid

Cost of performing an oil change (not including lubricant cost)

Accessibility of machine for changing the oil

OEM recommended oil change interval

Availability of oil analysis (based on turnaround time)

General cost/penalty of machine failure

Quality of filtration

How the Condition-Based Oil Change Tool Works

The tool deploys a scoring system that lets you simply complete a series of nine questions, which when totaled, give a composite score. If the score is below 100, condider using an interval-based approach. A composite score of 100 or more suggests that the machine is a good candidate to run oil changes based on-condition. A score greater than 200 gives near certainty to the decision.

Following is a discussion of the rationale of the nine scoring elements:

1. Lubricant Volume. If time and effort must be exerted to extend oil drains using a condition-based strategy, the total amount of lubricant involved needs to be factored in. If the extended drain saves very little fluid (small oil compartment) the cost may not be justified, even if the benefits were extended threefold. Read on, because there are other cost/benefit factors to consider.

2. Makeup Rate. Lubricant compartments that undergo high makeup rates due to leakage, oil burn or other causes of fluid loss may never need to be changed. The new makeup fluid not only introduces more additive to the system but also dilutes the concentration of contamination (water, dirt, etc.), acids and other degradation precursors.

3. Lubricant Cost. The more expensive a lubricant, the greater the motivation to save the oil from a premature drain. Many specialty and synthetic fluids can cost 10 times more than their less expensive counterparts. Because one of the driving factors for purchasing specialty lubricants is long life, a more precise decision in timing the oil change is justified in most cases. Reclaiming used oils is also a consideration here. The cost of reclaiming and reconstructing additives, when needed, is in essence an oil change but at a lower cost (potentially) than the physical replacement of the oil. A partial drain (bleed and feed) is yet another consideration.

4. Nonlubricant Costs. There are many cost factors in performing an oil change that go beyond the cost of the oil itself. These need to be weighed in terms of the benefit coming from more precisely selected lube change intervals using the on-condition strategy. For typical examples of such costs, refer to the above list in the article “Hidden Costs of an Oil Change”.(2)

5. Machine Accessibility. Many machine lubricant compartments, including fill and drain ports, are not accessible for routine maintenance. In such cases, the need for access must be planned well in advance, keeping the frequency to a minimum. The use of oil analysis to forecast oil change needs to avoid unnecessary change-outs could be of considerable practical benefit.

6. OEM Recommended Change Interval. Many OEMs have precise guidelines for when lubricants should be serviced. Others offer minimal advice. Because machine operating conditions (loads, speeds, temperatures, etc.) vary considerably, the service manual recommendation will often be on the far side of conservatism. This can lead to repeated premature interval-based scheduled changes. This part of the tool favors condition-based oil changes when the OEM advises longer change-out intervals. In such case, there is a more manageable timeline to monitor the oil and plan lubrication work orders.

7. Oil Analysis Availability. While oil analysis is highly accessible for most organizations, there are many instances when machine lubes cannot be sampled or convenient laboratory analysis is not available. This may be due to geography (remote locations of equipment), manpower issues or a host of other reasons. Clearly, the more frequently oil analysis can be performed, the more effective and dependable the on-condition oil change strategy can be. Onsite oil analysis offers clear benefits here but the cost and manpower to provide the service onsite can, in cases, be a significant offset.

8. Penalty of Failure. For machines that present high safety risk or downtime cost in the instance of operational failure, it may be prohibitive to switch entirely from interval-based programs. In these cases, the benefits don’t outweigh the risks. Often, when a high degree of reliability is desired, the best plan is a combination of condition-based and interval-based - the oil is changed whenever either first occurs. Because oils can occasionally experience premature failure or a wrong lubricant is introduced, such a combined strategy can make the best business sense.

9. Quality of Filtration. Proactive maintenance is what gives lubricants longer and more reliable service life. When lubricants are routinely kept clean, dry additives last longer and lubricated surfaces are less challenged (corrosion, abrasion, etc.). High quality contamination control improves the opportunity to extend and plan drain intervals on condition. In contrast, lubricants loaded with contaminants due to high ingression or poor filtration will often need to be changed to avoid distress to surfaces, regardless of whether other fluid properties have degraded. In such cases, the oil change becomes the default maintenance procedure for eradicating dirt and water from the machine - not exactly ideal, but often the only practical plan. Also worth consideration is the concern that new lubricants may not be much cleaner than the lubricant being replaced. Moreover, the cost of filtration is a factor as well.

In the past, many organizations have exclusively used interval-based oil change criteria. The interval was based on an assortment of considerations, such as the calendar, operating hours (meter), fuel consumed, miles/kilometers driven or production/work performed. In many cases, an approaching outage and shutdown have a driving influence on the decision, coming from the desire to avoid unscheduled downtime later or the need to change lubricants “on the run”. In addition, new equipment still under warranty may have OEM-specified lube change-out intervals which can make the matter far less subjective (and optimized).

The condition-based oil change strategy is indeed important in reducing oil consumption and associated costs, however, there are many situations with certain machines when maintenance and reliability are not “optimized” when the strategy is applied. In order to perform a condition-based oil change, there is added cost to monitoring the conditions, namely oil analysis. There may also be added risk from running the oil too close to end-of-life, which could lead to such challenges as the following:

1. A sudden and inconvenient need to change the oil.

2. Wear and damage to the machine.

3. The need to perform an expensive system flush prior to introducing new lubricants (resulting from oxidation of the belatedly changed oil).

Regardless, for most companies and machinery applications, the benefits of the condition-based oil change far outweigh the risk and cost.

Hidden Cost of an Oil Change

There are many hidden costs of an oil change that have influenced companies to find a more efficient approach to keeping oil healthy and reliable. A recent study on the subject found that the true cost of an oil change frequently exceeded 40 times the cost of the oil itself.2 Following are a few of the many factors that contribute to the true cost of an oil change:

Lost Production. Machine downtime and longer outage intervals needed to change oil, flush and prepare equipment to return to service.

Paperwork. Includes maintenance schedules, data entry, manpower planning, inventory management, work orders, documentation, etc.

Labor, Supervision, Benefits. The time required to drain a compartment, transfer lubricant and associated equipment to the machine, flush, fill and prepare the machine for service.

Handling and Storage Costs. For lubricants and associated equipment (filters, funnels, drum pumps, etc.).

Purchasing and Inspection. There are many real costs associated with purchasing lubricants. Also, new lubricant deliveries involve inspection, labeling, stocking and more paperwork. Inspection often involves lab costs.

Disposal Cost. Relates to solid and liquid waste resulting from disposal of oil, sludge, rags and cleaning equipment.

If it Ain’t Broke . . . Don’t Fix It. Often a lubricant remains serviceable for long periods of time without a drain and new lubricant charge (influenced by makeup rates, operating temperature, fluid volume, cleanliness, etc.). Draining an oil and adding new lubricants into a system presents risks such as:

1. Introduction of a wrong oil.

2. Introduction of a contaminated oil.

3. Introduction of an incompatible oil (with residual oil remaining in the machine).

4. Resuspension of settled contaminants in tank/sump floors and inactive zones.

5. Human agency failures (dead-heading pumps on restart, cleaning solvents not removed, introduction of contaminants, loosening of machine parts, etc.).

When compiled, with real cost and associated risk of an oil change factored in, it is often wise to let the conditions of the oil drive the decision.

Side Benefits to Monitoring Remaining Useful Life of Lubricant

Using oil analysis to monitor the remaining useful life of lubricants is often thought of as its primary application. However, as many proponents of oil analysis will profess, there is an assortment of other important benefits as well. After all, when monitoring the oil you should, at the same time, be monitoring other vital signs of machine health and oil condition including wear, lubricant contamination, wrong oil, lubricant compatibility, etc.

Not all in-service lubricants have a well-defined and predictable wear-out rate or mean time between failure (MTBF). The reason for such randomness stems from varying lubricant quality, changing contaminant ingression rates, application-dependent duty cycle, seasonal ambient conditions and other forcing functions of degradation. If an interval-based oil change criterion was used with a reasonable safety margin (confidence level: changing the oil ahead of actual need more than 95 percent of the time) then on average, many lubricants would be changed when the remaining useful life is more than 50 percent of the new oil. Naturally, such guesswork results in wasted effort, time, money and resources in the same proportion.

Other Factors to Reducing Lubricant Consumption

The act of effectively performing a condition-based oil change helps schedule the oil change with greater precision, such as not too late, not too soon (Figure 1). In essence, the oil communicates that it is “tired” and needs to “retire”, but we must be tuned to its message. While timing is important, perhaps more important is the strategy to proactively improve conditions that extend oil life. When applied correctly, a proactive maintenance strategy can double or triple lubricant service life. This is achieved by reducing the conditions that stress an oil (cleaner, cooler, dryer, etc.).

Prior Year
Year 1
Year 2
Year 3
New Purchases
10,000 gal
4,500 gal
2,100 gal
1,420 gal
Machine Volume Charge
4,200 gal
4,600 gal
4,600 gal
4,800 gal
Consumption Ratio
Consumption Ratio =
Annual Oil Purchases/Machine Charge Volume
Seek 50% reduction each year until a consumption ratio of less than 0.1 is achieved. Varies by industry and age of machine.
Figure 2. Trend Oil Consumption Ratio

Many people monitor improvements in lubricant consumption by monitoring the Lubricant Consumption Ratio (LCR). LCR is the ratio of the amount of lubricants and hydraulic fluids added to machines each year divided by the amount of fluids these machines hold (Figure 2). By applying oil analysis along with proactive maintenance, a significant downward trend in consumption ratio is generally achieved in the first several years. An example of how to achieve improvements in consumption ratio is presented in this Tutuka Power Station case study.


There are many factors and considerations to making the oil change decision. For critical machinery and in cases when the cost of the oil change is high, there is real need to challenge the norm and past assumptions. The Condition-Based Oil Change Tool offers a systematic process to efficiently use a customized weighing process to arrive at a near optimal decision.


1. Thibault, R. (2001, January-February). Converting to Condition-Based Oil Changes - Part I. Practicing Oil Analysis magazine.

Thibault, R. (2001, March-April). Converting to Condition-based Oil Changes - Part II. Practicing Oil Analysis magazine.

2. Brown, K. (1999, May/June). The Hidden Cost of Oil Changes. Practicing Oil Analysis magazine.

3. Cattaert, A. (2001, May-June). Major Reliability and Lubricant Consumption Savings at Tutuka Power Station. Practicing Oil Analysis magazine.