Don't get me wrong - oil cooling works a treat. It's just that the volume required is many orders of magnitude greater than required with ferrofluid and this creates unnecessary drag. Additionally, keeping the oil on the inside of the hub is a losing battle.
Ferrofluid contains nanomagnetic particles, far too small to abrase any of the motor components. The specific formulation is extremely important when it comes to this application. I'm fairly sure that the formulation used in 'Statorade' is Ferrotec APG1110.
Justin, the pioneer of ferrofluid hub cooling has tested motors for 10's of thousands of miles without adverse effects. The only concern might be the waterproofing measures you have already applied.
See the following thread for further information:
https://endless-sphere.com/forums/viewtopic.php?f=2&t=48753