FWIW, I have been using Redline's Diesel fuel additive. It comes in a bottle with a long tapered neck, and it's clear with markings on the label. (not sure how accurate they are, can check with an empty)
The diameter of the bottle is such that it fits (loosely) in the narrower far corner of the trunk pocket of my '10 Jetta, a small towel in there is enough to keep it in the corner, right side up. That stuff stinks pretty good, though a wipe with a paper towel at the station, and I've hardly ever noticed it in the car. Never leaked. I have no idea if the Opti-Lube stuff smells/permeates more.
I finally just broke down and ordered some Opti-lube SummerX, so I'll see how well reusing that bottle works. Think I'll need a funnel, it's a pretty tiny neck, but that's what makes it good for pouring. I'll be getting the Opti-lube 8oz bottle too, to compare.