Is Group II Base Oil Really the Cause of Varnish?

Varnish should not be explained by base oil group alone. Finished-lubricant performance and operating conditions both play important roles.

Common Customer ConcernsWhen varnish appears in a turbine, a common question is, 'Did this happen because we changed from Group I to Group II?' This belief may arise because the widespread adoption of Group II coincided with growing industry attention to varnish. However, a correlation in timing does not mean that Group II is the sole cause.

Technical Explanation

Group I and solvency - Group I oils contain more aromatic compounds and generally have higher solvency, allowing them to keep a larger proportion of certain degradation products in solution.

Group II/III and oxidation stability - Group II and Group III base oils have higher purity and generally better oxidation stability, but their solvency may be lower than that of Group I. Consequently, once degradation products form, they may come out of solution more readily under certain conditions.

Consider both formation and solubility - A varnish assessment must address two questions at the same time: how quickly the oil produces degradation products, and how effectively it keeps those products in solution.

The finished lubricant matters more than base oil group alone - Turbine oil contains base oil, antioxidants, and other additives. Deposit tendency therefore depends on the complete formulation, including thermal stability, antioxidant chemistry, air release, water separability, and operating conditions.

Impact on Machinery

Selecting oil by base oil group alone may not reflect actual performance
Compatibility and deposit control may be overlooked
A product may be changed without assessing residual oil and existing deposits
Oil life and reliability may fall short of expectations

Points to Consider

Select turbine oil according to OEM requirements and finished-lubricant performance
Consider operating temperature, duty cycle, and system history
Perform a compatibility assessment when changing brand or formulation
Inspect existing deposits before deciding whether to rinse or flush

How Can SUTAIYO and Lubefix Help?SUTAIYO can evaluate Mobil Lubricants against the machine's performance requirements rather than relying only on base oil group or ISO VG, while also considering compatibility with the existing system. Lubefix can support system preparation through oil draining, filtration, rinsing, or flushing when required before new oil is filled.

Customer Benefits• Lower risk from selecting a lubricant with incomplete information• Better conditions for the new oil at the start of service• Greater likelihood that the oil will deliver the required performance