See also:

Rear differential gear oil
There is no rear differential gear oil level gauge. To check the rear differential gear oil level, we recommend that you consult your SUBARU dealer for inspection. ...

Lubrication oil circulation in the refrigerant circuit
Operate the air conditioner compressor at a low engine speed (at idle or low driving speeds) a few minutes each month during the off-season to circulate its oil. ...

One Gear Shy
To solve the problem for good, and improve the car markedly, Subaru needs to replace the five-speed manual with a six-speed. So that you know where I'm coming from, I'm not someone who demands six ...