Users often complain that before using Group II base-stock oils, they didn’t have a problem with varnish. As a result, Group II oils have a reputation for being varnish prone, leading users to search for alternatives. The market has responded, with many companies offering low-varnish versions, co-base stock oils, synthetic oils or even after-market additives for the more adventurous.
The truth is, Group II oils do have less capacity to hold breakdown products in solution than Group I. This is because breakdown products are polar in nature and on the spectrum of polarity, Group II base stocks are less polar (more non-polar) than Group I base stocks. Therefore, because “like dissolves like,” polar breakdown products are more likely to precipitate out of solution from more non-polar Group II oils than Group I resulting in varnish deposits.
To understand how these oils breakdown over time, we ran thermo-oxidative breakdown experiments in the lab subjecting each oil sample to peak turbine operating temperature (150°C), air and a copper catalyst. The results presented include acid number, LSV amine and phenol antioxidants and MPC varnish potential (Fig. 1 & 2).
In the Group I sample, we see a rapid increase in varnish potential achieving a critical value in <5 hours. Acid number remains stable until approximately 80 hours when a rapid increase is observed corresponding with depletion of the amine anti-oxidants. While a Group I user may not change this oil because of MPC value, they would be forced to because of the significant increase in acid number. In the Group II sample, varnish potential values increase gradually, not hitting a critical value until 400 hours. Strikingly, we see no increase in acid number with amine life extended to 700 hours – 10x that of the Group I sample.
Overall, we see a Group I base-stock oil that is sensitive to oxidation that quickly reaches critical varnish potential values and a Group II oil extremely resistant to oxidation with no acid production over the life of the test, even after the antioxidants are consumed. While some users may discount this lubricant because of the varnish potential numbers observed, in actuality the performance is exceptional. Considering that resin-based varnish removal systems are very effective, the perceived limitations of Group II oils are misunderstood.