How "low on oil" did you find it? If it took 5L it either had none in it or you've overfilled it and hence the smoke.
I would also bet it's sucked some into the intercooler.
If it was 1/2 full 3 weeks ago did you top it up? Though even if you didn't, to use that much in that time is definitely not right. Sounds like something is broken inside...ringland, ring...etc.
If it is under any sort of warranty you should get it back to where you bought it quick smart....

__________________
No Subarus currently owned