The Importance of Viscosity Index - A Case Study

Noria Corporation

A client recently contacted Noria wanting to identify an alternative lubricant for an older piece of equipment. The original lubricant suggested by the manufacturer was vague and specified a naphthenic mineral oil. The client wanted a high-quality mineral or synthetic oil to protect its asset.

The Facts

The unit of concern is a right angle gearbox containing brass components. The OEM recommended “naphthenic base oil with a viscosity of 85 to 95 SUS at 210°F”. Naphthenic base oils have changed over time and it is important to note that this piece of equipment was built in 1965. The average operating temperature of the gearbox is estimated to be 60°C to 70°C.

The possibility of water contamination is low; therefore oils that are sensitive to water contamination can be used. The brass components in the gearbox require a noncorrosive lubricant. Oils containing sulfur and phosphorus additive should not be used. HiPerSyn 150 was previously recommended to the client from another source. HiPerSyn 150 matches the 85 to 95 SUS at 201°F, and from this standpoint it appears to be a good match.

Another gearbox in the same system is using Ultra Gear 320. The client would like to use Ultra Gear 320 to consolidate lubricants. This would also reduce the possibility of cross-contamination.


The original lubricant specified was a 1965 naphthenic base oil with viscosity of 85 to 95 SUS at 210°F. It was determined that the viscosity index of a 1965 naphthenic could have varied from -30 to 70. This information is based on articles from that time period. Using ASTM D2161, it was determined that 85 to 95 SUS at 210°F is equivalent to 17 to 19 cSt at 210°F. To reduce the variability, 18 cSt was used as the viscosity at 100°C.

As stated above, HiPerSyn 150, a synthetic base oil, was recommended to the client. According to the product data sheet, HiPerSyn 150 has viscosity of 143 cSt at 40°C, and 18.3 cSt at 100°C. The viscosity index is stated as 144.

Ultra Gear 320 is a mineral oil containing a borate extreme pressure additive package, and it is currently being used in a different gearbox nearby. According to the product data sheet, Ultra Gear 320 has viscosity of 320 cSt at 40°C, and 24 cSt at 100°C. The viscosity index is stated as 95. Table 1 presents a summary of the lubricants.

Table 1. Lubricant Summary

Analytical Tools

A viscosity index chart is used to compare the possible alternative lubricants to the originally specified lubricant. This chart enables the full viscosity profile of the lubricants to be observed and compared. It is an invaluable tool in comparing lubricants. To plot the viscosity profiles, two viscosities at two temperatures are needed.

This is available for both of the alternative lubricants, but for the original lubricant only one viscosity at 100°C has been given and the viscosity index has been estimated. ASTM D2270-04 will be used to calculate the viscosity of the naphthenic base oil at 40°C. Using formulas from ASTM D2270-04, the viscosity at 100°C of 18 cSt, and the viscosity index range of -30 to 70, it was calculated that the viscosity at 40°C would have been 260 cSt to 472 cSt, respectively.


HiPerSyn 150 is a synthetic that would provide better oxidation stability. At 100°C, both HiPerSyn 150 and the OEM naphthenic share the same viscosity of 18 cSt. At 40°C, HiPerSyn 150 has a viscosity of 143 cSt and the OEM naphthenic has a viscosity of 260 to 472 cSt. HiPerSyn 150 is considerably less viscous than the OEM naphthenic at 40°C, which can be seen in Figure 1. In the operating zone (60°C to 70°C), HiPerSyn 150 consistently has a lower viscosity than the OEM naphthenic.

Viscosity Comparison of Ultra Gear Oil 320 and OEM Naphthenic Oil

Ultra Gear 320 is an extreme pressure (EP) mineral oil that utilizes borate additives instead of sulfur and phosphorus additives. Borate is a noncorrosive additive; therefore it will work with the brass components in the gearbox. Borate additives have poor performance with water contamination, but water contamination in not a concern for this gearbox.

Ultra Gear 320 is currently in use on the same system in a different sump. Utilizing Ultra Gear 320 will consolidate and reduce cross-contamination. At 100°C, Ultra Gear 320 is 24 cSt and the OEM naphthenic is 18 cSt. Ultra Gear 320 is more viscous at 100°C than the OEM. At 40°C Ultra Gear 320 is 320 cSt and the OEM naphthenic is 260 to 472 cSt.

Ultra Gear 320 is within the range of the OEM naphthenic oil. In the operating zone (60°C to 70°C), Ultra Gear 320 is within the range of the OEM naphthenic oil. Figure 2 compares the viscosities of the two oils.


HiPerSyn 150 would provide excellent protection when applied to the correct application. This, however, is not the correct application. HiPerSyn 150 shares a common viscosity at 100°C with the OEM specs, but the similarity ends there.

Throughout the operating zone, HiPerSyn 150 has a lower viscosity than the OEM specs, which could lead to excessive wear. HiPerSyn 150 is not recommended for this gearbox.

Ultra Gear 320 is a good-quality lubricant with EP additive including borate. The system is not susceptible to excessive water contamination, so the borate additive will work. Ultra Gear 320 has good viscosity throughout the operating temperature and it falls within the original OEM specs. With the higher VI provided by Ultra Gear 320, there should be greater protection in temperature extremes.

At low temperature, Ultra Gear 320 will be less viscous than the OEM spec, and at high temperatures it will be more viscous than the OEM spec. Utilizing Ultra Gear in this gearbox will consolidate lubricants and will reduce potential cross-contamination. Ultra Gear 320 is recommended for this gearbox.

A few important lessons have been learned from this case study. Don’t select an alternative lubricant by matching only one viscosity point. HiPerSyn 150 initially seemed like a good match, but was proven otherwise.

Ensure the lubricant additives are compatible with the parameters of the gearbox. By ensuring that the borate additive was compatible with the gearbox, possible side effects were eliminated.

A viscosity index chart provides the full viscosity profile of a lubricant and is an excellent tool for comparing lubricants. It showed that the HiPerSyn 150 was a poor match. When selecting an alternative lubricant, the viscosity should match the OEM recommended viscosity in the operation zone.

The viscosity index chart showed that the Ultra Gear 320 viscosity was within the allowable viscosity range in the operating zone.

Selecting lubricant that is currently used at the site is best practice. By selecting Ultra Gear 320, the client is able to consolidate lubricants and prevent possible cross-contamination.


  1. ASTM D2161: Standard Practice for Conversion of Kinematic Viscosity of Saybolt Universal Viscosity or to Saybolt Furol Viscosity.

  2. ASTM D2270: Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C.

  3. Chevron HiPerSYN Oils Product Data Sheet. April 11, 2005. ChevronTexaco Product Company. March 20, 2006.

  4. Chevron Ultra Gear Lubricants Product Data Sheet. July 19, 2005. ChevronTexaco Product Company. March 20, 2006.

  5. “Oil for Lithium Greases.” Lubrication L1.1 (1965): 1-8.

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