RPVOT 40% is a warning for you that the service life of the lubricating oil was consumed and you need to do something if you want the existing oil to still servicing more further. Otherwise soon you have to schedule it for Oil Replacement.
Before Top-Up action of 30 - 50%v, Since it is 6 years servicing already, then I recommend you to check The Varnish Potential Rating Test by MPC (ASTM D7843), and RULER Test as well (ASTM D6971).
Once OK, go ahead for Top-Up. IF NOT, (1) it is recommended for you to reduce the varnish rating using varnish removal devices. (2) Oil replacement.
Good luck Bai.
RPVOT measurement is an indirect method of measuring Oxidation stability of turbine oils with a lot of variation in RPVOT results between labs, testing equipment and the procedures used by the lab operators.
The accuracy of RPVOT test is almost +/- 40% of the results.
On other hand, there are some direct methods available for measuring Oxidation stability of turbine oils.
Coming to the failure mechanism associated with low RPVOT results, it is actually lubricant Varnish deposits which formed when the oil had low RPVOT, rather than the low RPVOT on its own.
This point is supported in ASTM 4378-13 (Standard Practice for In-Service Monitoring of Mineral Turbine Oils for Steam, Gas, and Combined Cycle Turbines)
I would suggest you to read my article on RPVOT on Linkedin at below link.
Feel free to drop me a message at firstname.lastname@example.org in case you need any further clarifications.