Trans output shaft seal - why is this groove there?

[gregb]

My bike has been losing some oil from the drive shaft and it's going into the transmission. So while I have the trans out I thought I would change the output seal, since that would seem to be the culprit. But now I'm not so sure.
Firstly I don't see why it would move from drive shaft to transmission as seems to be a common thing. But the spring on the output seal is on the drive shaft side, indicating that's preventing oil going into the transmission. Can anyone explain what's going on here?
Also, at the top of the seal housing is a groove. I thought this was odd but it appears in other photos so it's supposed to be there. If I turn the trans upside down on the bench, oil comes out. (see photos) So that tells me it's above the normal oil level in the transmission. But the oil in the drive shaft will be flung around violently by the universal joint so what's to stop it getting into the transmission through the groove?
Don't know if I'm missing something here but I can't see why that groove is there, other than to cause oil migration.
If anybody can shed light on this topic it would be greatly appreciated. I'm now tempted to not change the seal, and why wouldn't one block that groove anyway?

[dogshome]

Totally uninformed guesses 

Looking at pis, the early seal on the box is designed to keep oil in, not to keep it out. The lip appers to point inwards. So any oil in the driveshaft will have an easier time getting in than out. But then it look like later seals are reversed (as you said, spring outside).

The groove is a breather to stop oil being forced one way (or another) through the seal by pressure buildup from heat in the driveshaft. Likewise when contracting when cooling. The gearbox has it's own atmospheric vent.

Of course the bevel gear for the speed is whizzing away behind that seal, it's a helical oil pump! Which way does it rotate and it it flinging oil towards the seal?

[gregb]

Thanks for your reply. Not uninformed guesses though, you're right I think. It's initially confusing too with the seal being reversed on some models.

Been doing a lot more reading since posting and while that groove is to equalise pressure in the trans and drive shaft, it's a funny way to do it. Even more odd is the vent in the transmission. A hollow bolt that holds the speedo drive and also the battery negative lug - I don't think I could dream up a more bizarre and potentially problematic way of implementing a vent. Those Germans!!

So anyway, I don't think my output shaft seal is at fault and this is just the way it is. I'm pretty certain that if you are going downhill (or even not) and slam the brakes on, that nice warm and thin driveshaft oil will rush forward and flood the output seal area and some will slosh through the groove. And then eventually enough gets into the transmission to make you think there is a seal problem. That's my theory anyway.

If I didn't love riding this thing so much .... Some of the engineering on this bike impresses me but I do scratch my head a lot too.