I've PM'd people who bought this lens new. It wobbled for them. I've Googled a variety of threads in forums on the topic, almost all of them wobble - new or refurb. A few started out tight and then began to wobble. I saw two posts from people who said theirs were still tight. I saw one post from someone who returned theirs because it was so tight the lens squeaked when they zoomed.
It appears less than 1mm of wobble when extended to 40mm is pretty normal, something around 0.5mm being typical. This displacement being measured at the filter rings while pushing the end of the lens barrel to force it to deflect.
Mine wobbles by just about 0.5mm, perhaps a hair less. As I rotate the lens or lens and camera I can hear it make a small "thunk" as gravity pulls the end of the barrel down into a new position. If I shake the lens like in the video in this thread mine makes the same wobbling/thunking sounds.
I've tested my for decentering comparing all four corners shot on the same resolution target at 40/2.8 and I've done this with the camera in three orientations (landscape and portrait turned both left and right). In all cases the four corners are very sharp. There is one corner that at 200% is just barely visibly less sharp than the others. In my experience that is pretty typical, I don't own a fast prime that doesn't have one corner very slightly softer than the other three. This "soft" corner does not move relative to the sensor position when I shoot at different rotations so it would appear to not be related to the wobble at all.
As to weather sealing it is all done with rubber gaskets, rubber is flexible and so it should handle the small displacements at the junction just fine. If the end of the ends barrel is displacing by 0.5mm then at the junction with the outer barrel it is displacing a whole lot less than that (simple geometry). Based on what is written and shown in figures on the Olympus site air is not designed to pass between the inner and outer barrel junction so I suspect that gasket should be plenty large enough to handle this very small play. Air enters the lens under the zoom ring itself actually.
Based on all this I'd say if you've got a lens with wobble of less than 1mm then there is little point in returning for an exchange or buying a brand new one. You will more than likely just get another that has a similar wobble perhaps of slightly different magnitude.
Clearly there can be too much wobble and if there is a lot of displacement return it. But my copy makes all the same rattling and thunking sounds when shaken as in the video in this thread and optically it tests beautifully. I can't from that video at all tell how much the barrel displaces because when he presses on the barrel he also moves the lens body, so I don't know if that lens has more wobble than mine or not. I measured mine against a stationary ruler suspended just above the end of the lens while holding the lens mount flush on a table and pressing on the side of the extended barrel. If you have a dial gauge that would of course be more accurate, I didn't feel like looking through boxes to find my old one so eyeballing with a ruler was sufficient to determine it was a bit under 0.5mm.