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.