This is the best grease for those caps obviously but those Dutch dwarves are slippery little buggers aren't they. I suppose that's why those caps are so expensive. Then, of course, due to their moist environment they do suffer from foot rot which slows them down a bit.
I reckon it's worth asking them if they sell rubber sealings to put around hubs.
Actually thinking about it I'm sure
@dancing james puts aftermarket rubber sealings on his Chris Kings. Maybe drop him a PM.