A few questions.

Did you put silicon RTV on the outside of the axle seal before installing?
Are you sure the axle seal is leaking and not the "finned" retaining nut?
Do you park on a hill/incline regularly?

 

A few questions.
Did you put silicon RTV on the outside of the axle seal before installing? No
Are you sure the axle seal is leaking and not the "finned" retaining nut? I don't it's the finned sundial as the leak got way worse with the recent oil seal
Do you park on a hill/incline regularly? No

 

Clean and monitor. Could be oil you put on the seal, did the fsm say to put oil on it? Some seals you don't want to put oil on the outside, it creates leak paths. And some even have a sealer on the outside. Some even have small amounts of grease in them that heat up and leak out for the initial run in.

 

Interesting point, maybe I misread the FSM (image below). It does say to apply oil to the oil seal lips but this is after lightly tapping the seal into place, think I oiled both sides in the end rather than just the part where the axle enters.

 

Here's the FSM page, steps 1 to 7 describe filling the gear oil so are not included here. .

 

When I did mine, I tapped them in with the outside dry and put some synthetic grease on the inside and also on the spring. I use a really sticky grease called Green Grease. Coating the spring with it helps reduce the risk of the spring popping off during install. Just coat around the outside with it already in place, don't remove and grease the whole spring.

Perhaps the spring came off/broke or there's something wrong with the axle stub that chewed up the new seal.

 

You might have receive the seal for the other side, which can make the leak much worse. I discovered that Subaru had a batch of seals labeled incorrectly when I changed out the left front axle shaft on a friends car.

If you look at the seal, there are small ribs just outboard of the seal lip that are angled so that whatever oil gets out past the lip gets pumped back towards the lip, which in turn blocks any more oil that is trying to get past the lip.

 

You might have receive the seal for the other side, which can make the leak much worse. I discovered that Subaru had a batch of seals labeled incorrectly when I changed out the left front axle shaft on a friends car.

I recently replaced these front seals on our Crosstrek & the pair I ordered from Subaru were just thrown in the box loose. Those small ribs were mirror images, so I knew they gave me one left & one right, I just didn't know which went where until I visually matched the old (original) removed one to the new one. I would be pretty peeved if the manufacturer mislabeled & caused me to repeat all that work.

If the old seal was original from the factory you could match it to your new seal to be sure they are identical, as opposed to the ribs being angled differently.

 

Thanks for these helpful posts I've learnt heaps about the seals and differential with progress. The axle stubs are good, relatively new, the seals supplied by Subaru were individually packaged with the correct part number. So I went ahead and changed the seal again tonight to find after a test drive there is a very small drip of oil on the LHS again but is better than before. I've learnt that a seal remover tool is worthwhile and that parts store seal drivers are not suitable as they don't go over the lips of the Subaru seal. Also that it's a surmountable challenge to do this work with the car on jack stands, the H6 exhaust pipes and LCA in place. It is possible to remove the axle easily after removing just the axle nut, brake caliper, two big strut bolts (mark the camber position) and ABS sensor. The knuckle then folds out nicely. Both LHS and RHS seals have ID 35mm, OD 50mm at 9mm deep on the mating surface but are appropriately directional with small ribs on the inner circumference, I didn't realise the reason the part numbers were different left and right was they are directional until reading the replies here.

I think I have two problems, the first being that the bore of the diff retainer where the seal snugs into has a scratch (just noticeable by feel with an un-gloved fingertip) likely from somebody in the past - prolly me Is the scratch fixable? I suspect not, sanding or polishing might only make things worse with a more uneven gap and metal dust getting everywhere but have no experience there. I see there is a similar post here.

The second is that not having the special seal driver tool described in the FSM, it's dimensions or experience of this work aside vicarious YouTube vids I fashioned an amateur seal driver from a bottle cap but the walls of the cap weren't wide enough. So I pushed the first seal in a bit too deep due to the driver following it into the bore also the seal didn't seem go in exactly square as a fingernail depth check felt that one side was in fractionally further. The second time today was better although just out. Approximate measurements require the driver to have ID of 46mm to go around the seal front lips (which require a minimum clearance of 4mm depth) with an OD of say 53mm to ensure the seal ends up flush with the housing // next time.