I recently bought it in hopes to remove some of the oxidation from a vulcanite stem. There's a minimal improvement, but it remains fairly brownish. The protection effect seems on par with mineral oil or other options. You still have to reapply it regularly. If you have mineral oil at hand, I'd just use that. Otherwise it's a nice handy small pipe-themed bottle that makes application easy.

I haven't had a lot of luck with either Obsidian Oil or plain mineral oil, but I know a lot of people have been successful with using such "preservatives" - after lots of time spent cleaning and buffing out the oxidation.

(TBH, I much prefer having a sturdy, beautifully colored acrylic stem on my pipes.   Since I don't clench my pipes, there is no advantage to vulcanite stems for me, and I find their high maintenance demands quite annoying and impossible to satisfy without a proper buffer.   My pipes with vulcanite stems are in a box in a closet where they can keep their ugly-looking, oxidized stems to themselves.  It seems counter-intuitive to use vulcanite, a material that needs to be protected both from the acidity of saliva and from any source of ultraviolet light, for a pipe stem.)

Nobody told me as a novice pipe smoker that I had to care for vulcanite stems so mine got really nasty and green with oxidation. Sodium percarbonate helps take the oxidation off, you can get it relatively cheap for homebrewing purposes. Soak it overnight and then use a magic eraser dipped in some ethanol to scrub off the oxidation. I use paraffin oil (i think that's the same as mineral) and I haven't had much oxidation build back since scrubbing it all off. I just wipe the stems with a papertowel soaked in vodka after each smoke and or the magic eraser if I was too lazy to clean it right after smoking. Of course without a proper buffing they look far more dull than before, but that really doesn't bother me as long as they taste neutral and don't stink like burnt tires. 

Obsidian oil isnt going to remove oxidation.  Its my understanding that its main purpose is to prevent it.  If its already present it will make the stem look better but if you want it jet black you will have to do some restoration.  There is really nothing that doesnt require elbow grease to get rid of oxidation on a brown stem.  If you are not using your pipes and they are ebonite low light is best.  Ebonite is photosensitive. Obsidian supposed to have UV blockers.  I dont know how well it works.  Its not sold in Europe.

I took a bunch of my eBay bought used vulcanite stems and soaked them overnight in an oxyclean solution. Took all of the brown residue off but of course removed the shine. I then applied obsidian oil to them after I cleaned up any chatter marks with some high number wet sanding pads. They came out like new.

I don't clench but put rubber pipe bit covers on them all just to keep them nice.