Starter motor on it's way out?

[Throbbinhood]

Is there any oil on the back face of the harmonic balancer?
Tell tale sign harmonic seal worn.
If it's only on long trips, it's the oil getting hot/thin enough to pass a seal one would think.
What brand & grade oil are you running?.

but your statement, Emptying out, sound like more than a drip, whats the go there ?

Haven't had a look at the balancer to be honest, but thanks for the tip, I'll check it out! There was a decent size patch of oil on the diff as a result.
As for oil, I'm running whatever was in the factory manual. Can't remember off the top of my head though.

If it was coming from the front of the engine, the timing belt would surely be soaked or at least show signs of contamination. and since there is no seal between the belt and balancer there would be oil or oil soaked dust on the bottom of the cover.

I'll have a look at this. Hopefully not much work to get the cover off. Would the oil soaked dust be on the outside of the cover? Or would it pool inside of it?

Well, if I hear you correctly, I would stop driving it right now and get to the bottom of it.
Oil vanishing into thin air is a serious concern for a relatively younger engine.

I would drain the oil completely and have a good look at the oil, whether it has too much soot in it or not.
Use 20/50 and see how it goes. I remember during central oz trek, mine did the same thing when I put 10/30(or 15/45). By same thing, I meant oil level dropping rapidly without any leaks. I experimented with 15/45 during this summer for local errands. The oil level did not drop, other than blowby getting worse. So, I reckon worn out engine/thinner oil/higher rpm longer drives is not a good combination.

It's not my daily, it's been sitting in the backyard since that trip - barring the drive to the carwash and around the block trying to get it to leak.
It's not vanishing into thin air though. The oil level on the line isn't dropping since I got back from Toolangi - and the slight drop in level on the dipstick for that trip I'd account to the leak.

And if you happen to be doing the front crank oil seal, make sure to apply enough HTB on the lips of the seal as well as the shaft, and be gentle with it. I had to do the whole damn thing all over again 'cos the seal lips went in crooked. Watch using a mirror to make sure seal's properly in, but not too far in by forcing it too much.
And it's probably a good idea to go for the original Nissan oil seal.

Thanks for the tip! Hoping it doesn't come to that, or, well, whatever ends up being the easiest solution anyway haha.