RTC3511 is the seal that fits inside the back of the hub, and keeps the grease in.
If you've got EP90 drippping down the brake disc, there's another seal inside the stub axle that stops the EP90 getting into the hub, where the grease should be. I understand some people omit to fit this to allow the axle oil to provide lubrication to the wheel bearings, but if you've not done this, and you're sure it's EP90 that's on that disc, then I think you might be looking at replacing two seals, plus probably the bearings..?
Kev