Short version: As I told a mate who recommended swivel grease because it doesn't leak - sand wouldn't leak either..
Long version: I rebuilt both swivel hubs on my 2A recently. Swivel hub refurb kit from Dingocroft, new seal, retainer, bearing, shims, etc. And I cleaned up the housings to see how they would do..
The side that was less corroded to begin with had oil in which I drained before the refurb. After refurb, this side still holds oil, it's full of EP90 and does not leak a drop (at rest).
The other side, that was more corroded to begin with, had thick black swivel housing grease in it (I suspect this was the one-shot stuff mentioned above as there was an unfinished bottle of it in the back when I bought the landy). It doesn't drain like oil does, so nothing came out the drain plug prior to removal. Inside it had flung out to the sides away from the universal joint anyway, so it was not lubricating or taking heat out of anything. (Still, previous owner probably put it in, thought "hoozah, no more leak!", and went off with a whistle..)
My 2p is to live with the leak, because knowing you have a problem is better than hiding the fact you have one - or fix it properly.
I'm going to put a new housing on my side that is still leaking, because I know from the other side that they can and will hold EP90 if parts are in good condition. And EP90 actually lubricates the UJ when it's in there, and can be drained and replaced.
If mine was in daily use and I couldn't get or afford a housing any time soon, I'd put a new seal on it and see if it would hold EP90 for a while before the seal started to go again. I'd rather keep aware of the issue than put the grease stuff in it and forget that it wasn't actually doing anything useful in there..
Contentious topic! Always a balance between "mine doesn't leak cos it's all brand new and sits in my garage", and "mine leaks, don't they all? I use it everyday and it's fine"