Concentric,
Further to my previous about the different oils - when using "mineral" oils the service period of flight hours and calendar period is much shorter.
That could easily contribute to a large proportion of the better result.
WRT your comments about "silica" - could it be something as simple as a poor vent on the gearbox allowing it to enter from just airborne debris? Dumber things have happened, wood for trees etc.
I did point out and demonstrate to a certain associated manufacturer how water was getting in to their gearbox which resulted in numerous changes to the venting and fixed the issue.
The unusual thing was that if you had any experience with older aircraft and different manufacturers the fix was obvious! No need to re-invent the wheel!