On my 2003 it extends a bit further than the factory drain plug. Somewhere between 1/8th and 1/4th inch would be my guess.
If you're really concerned, you could CAREFULLY notch the threads with a dremel to allow for the oil to drain better. That being said, when I pulled my Fumoto to replace the crush washer, all I got out was somewhere around 1/4th to 1/2 cup of oil, not enough to concern me.
No matter what you do, no matter what plug you have there, there will always be some "old" oil left, unless you let the oil drip "for ever" as mentioned in that thread.... I wouldn't worry about that....
I have used the supplied washer... I must have bought couple of dozens of them over the years and it was the best thing I've ever done. No more box wrenches! Just turn the valve, drain used oil into a plastic container and drop it off at Walmart's recycling auto center....
+1 on the supplied washer. The only "reason" I can think of is it's not metal and won't create any dissimilar metal issues. In reality I had a choice, a used crush washer and the supplied one--I used the supplied one.
The thread linked above talks about the F-105 valve, which is for pre-2010 Outbacks. Anybody with a 2010+ should be ordering the F-108 valve, per the "Sizes and Applications" page here: FUMOTO ENGINEERING
From the Sizes and Applications Page:
F-108: Outback SUV 4 cyl. Non-Turbo('10 & on), Forester Non-Turbo ('11 & on, Legacy 4 cyl. Non-Turbo ('10 & on)
I put notches about 3/16" deep in the top at 90 degree intervals around the threads. Then, the next time I changed the oil, after it drained, I removed the valve. No additional oil drained out. I reinstalled it, confident that ALL the old oil that can come out, does come out. Anal, but thorough......That is me!
Aren't these notches an overkill? If you change oil every 3000 miles (which is an overkill as well) then some residue of old oil won't make such an impact on the engine lubrication...just a thought....