I would like to add one important piece of information that was missing from the article. Namely, the API Groups use viscosity index as a major defining characteristic. However, most of the industry either continues to ignore or does not know the major deficiencies of the V.I. scale. It is not a technically consistent nor reliable indicator of its intended purpose-it does a poor job of rating the viscosity-temperature behavior of oils, particularly at low viscosity.

For this reason, any revision of the API base oil classification system should adopt a more scientific parameter for base oil characterization and not continue using V.I. One possibility, which is very easy and minimally disruptive, is to correct the underrating in the V.I. scale for oils with low viscosity. This can be done using a simple linear equation to increase the V.I. rating for oils with viscosities below 5.5 centiStokes at 100 degrees Celsius. A summary of work I have done on this topic was reported in the October 2013 issue of LubesnGreases.

Another possibility, which I discussed in SAE Papers 2012-01-1671 and 2015-01-2048, is to use the more consistent viscosity data from Fenske et al. for the 100-V.I. reference oil series and to disregard the 0 V.I. series.

Jack Zakarian

JAZTech Consulting

Orinda, California