VI and longevity of oils

Im assuming this is a gearbox? I don't think you can generalise on this question. There are many other factors that need to be taken into consideration -
is the equipment running constantly or intermittently, are there known problems (temperature, wear etc), what are you trying to achieve with a higher vi (better startup, higher range etc), have you tried hi-tac or synthetic forumulas, what is your OEM spec, are the operating environments unusual in temperature? etc etc etc...
Perhaps you could be more specific.

Speaking generally about hydrocarbon based lubricants, high VI oils are usually group III mineral oils or group IV synthetics which are naturally longer lived than the old group I mineral oils. These oils have fewer imputities and have a more stable molecular structure. By selecting a lubricant with a higher VI you should automaticly get longer life.

As a rule of thumb our experience has shown that the oils with the higher VI index usually perform better. I don't know if this has anything to do with the VI number itself or if it is just a result of the oil being grade III base stock or a PAO. We use the VI number as a tie breaker when looking at oil options. The one with the higher VI, all other things being equal, is the one that we will pick.

Hi Pat,
we make passive EP system lubricants and the POA base will be best,
keep in mind heavy chemical EP oils will react the additives shorteneing lubricant life so if you can look for PAO gear oils with solid boundary EP additives, not Moly (MoSO2) as the SO2 can react with water.

Thanks everyone for the superb help - this has answered my query perfectly. I now know which way to go.