lorain's guess makes sense to me.

Either way, 3 oz of oil isn't going to make much difference either way.

I don't really understand the "saving" it aspect; if you're anal (like many of us) you'll pour in 8.5 USQ and have a half-bottle hanging out in the trunk. I do know some who just dump in 8, and then add a quart when it drops, though. Again, that 3 oz won't make much of a difference...even assuming it does come out of the bottle.