There are esters and there are esters – the company I worked for made 80 different esters, including about 50 different POEs, all designed for lubrication. The beauty of ester chemistry is versatility, that is, the ability to design molecules with specific properties for specific applications by changing the ester molecular structure utilizing the broad choice of available raw materials. In addition to being able to vary physical properties such as viscosity, VI, volatility, lubricity, pour point and flash point, one can also vary chemical and performance properties such as oxidative stability, coking tendencies, hydrolytic stability, biodegradability, additive solubility, and seal compatibility. This versatility allows custom designing base oils, hence the great number of available esters.
Having been retired for nearly 16 years, I don’t know which esters are being used in today’s formulations. Traditionally PCMOs used POEs with high VIs and lubricity such as TMP C8C10, mainly as adjusting fluids for additive solubility and seal compatibility with PAOs. With the availability of lower cost, more polar Group III “synthetic” base oils, high cost POEs are not as needed as with PAOs, and have been replaced to some extent with alkylated naphthalenes (ANs).
Regarding HPL's oils, and for that matter all other brands, I do not know the formulations and have not seen comparative engine and fleet data, so I have no basis for a meaningful opinion on their relative performance and value. Like most people I am not able to scientifically determine oil performance differences by driving my vehicles or using UOAs, and so tend to rely on approvals and third party certifications. In the case of specialty oils, however, these may not be feasible and may have to be replaced with bench data and trust in the formulator. Dr. Rudnick is certainly an experienced and respected formulator.
Regarding hydrolytic stability with esters, this is not generally a problem in the field, partly because POEs are often used in high temperature applications where water does not exist, such as jet engines and industrial curing ovens. While the ester molecule can be “un-done” back into acids and alcohols, the conditions to do so are rather severe. Esters have been used in PCMOs for 50 years and hydrolysis has not been a significant field issue, but that may be different with alcohol fuels such as E85. I have not seen data on ester performance with alcohol fuels so I can’t comment on this.
That said, if hydrolytic stability is an issue it can be controlled with ester structure. By placing molecular branches near the ester linkage one can sterically hinder the reaction with water, even to the extent of matching hydrocarbons in hydrolytic stability. Such chemistry is common in POE refrigeration oils, but I don’t know if it is being used in motor oils. The downside is a lowering of VI and some impact on low temperature flow, both of which can be mitigated by reacting other acids onto the molecule.
Bottom line is that esters can be structured to provide a balance of properties optimized for a specific application. Unfortunately many formulators do not take advantage of this versatility because it requires a close and trusting relationship with the ester supplier and an extensive testing matrix. Easier to just test the ester the supplier offers or select from a physical property table.
For a more detailed discussion of esters and their properties, see my paper on the subject here:
Esters in Synthetic Lubricants