Extensive idling may lead to fuel dilution, but high-loads may cause shearing as well.

Measuring both the 40°C and 100°C viscosities allows calculation of the viscosity index (VI). The VI of an oil relates to the stability of the oil's viscosity against temperature. It is a unit less number of about 100 for mono-grade oils, around 140 for SAE 15W-40s and >180 for SAE 0W-40s. Higher VIs are achieved using premium base oils and a VI improver additive. The defining difference between mono-grade and multi-grade, as prescribed by the SAE J300 standard, is principally achieved using VI improver additives.

This additive is a long-chain polymer that contracts at cold temperatures and relaxes or unfurls at higher temperatures, which offsets the expected decrease in viscosity when oil is heated. The advantage to using VI improvers is that oils may operate in a wider temperature range, but the disadvantage is that this molecule is susceptible to mechanical shearing. For example, an SAE 15W-40 oil is formulated using an SAE 15W base oil, but with the additive, it will meet the viscosity requirements of an SAE 40 oil at 100°C. However, since no SAE 40 oil is used in the blending, the oil reverts to nearly an SAE 15W oil as the oil is sheared, possibly leading to premature failure due to an insufficient lubrication film.

Shearing is very common and even expected in most multi-grade applications, thus the tendency for oils to be formulated towards the thicker end of the allowable range. However, extreme conditions (load, environment, temperature, speed) or extended intervals may cause excessive shearing to the point where the oil falls out of grade and thereby voids warranty claims and leads to excessive wear or possibly failure.

The best way to monitor used engine oils for adherence to grade is to monitor the 100°C viscosity, noting any significant decreases. However, shearing does not cause all decreases. Water/ fuel contamination or mixing of products also may be the culprit. In addition, increases in the viscosity due to soot contamination or general oxidation may mask shearing. The only way to be certain that shearing is not excessive is to include a 40°C viscosity measurement, which shows larger numerical deviance.

Having both the 40°C and 100°C viscosities allow calculation of a conclusive VI. No matter how much contamination is in the oil, the VI will not change without something happening to the VI improver first. So even if the 100°C viscosity remains in grade, it will still be possible to see if the oil has suffered significant physical damage.

However, another error may be compounded into the report if fuel dilution is correlated from the viscosity results as well. This correlation often assumes a fixed viscosity for fuel, which is reasonably fair if the decrease in viscosity is only due to fuel and not shearing, and if neither soot contamination nor oxidation are masking the results.

Let's examine two examples using the same measured 100°C viscosity, but one assuming VI is constant and calculating the 40°C viscosity, and the other with a measured 40°C viscosity.

The results in Table 1 suggest an approximate 1% fuel contamination in the calculated example, but none in the measured example. While the fuel level is not elevated enough to suggest an immediate corrective action or mechanical issue, it is misleading nonetheless. More importantly, the true measured value shows significant shearing, suggesting the need for an oil change and possible reduction in future drain intervals.

The results in Table 2 suggest a minor decrease in viscosity in the calculated example, but significant shearing in the measured example. The viscosity decrease in either example is not enough to suggest an immediate corrective action. However, due to masking by either soot contamination or oxidation, shearing is evident when the 40°C viscosity is measured and becomes obvious when the VI is included.

Going forward, it is imperative that end-users ask their labs precisely what is being provided to them. Both viscosities must be measured and reported in order to maximize the value of routine oil analysis, and having the VI reported enables faster and more correct interpretation.

Evan Zabawski is a Fourth Class Power Engineer with a diploma in chemical engineering. Prior to joining Fluid Life, Evan gained previous experience as a manager of a tribology lab before moving into the field of power generation fluid maintenance. www.fluidlife.com