Honestly, I think you're just delving into semantics. If you consider 'instantly fatal' to just mean fatal within exactly 0 seconds of the act occurring, then a lot of things that I personally would consider 'instantly fatal' wouldn't be, and that's not going into the prickly thornbush that is a unambiguous definition of death.
Actually modeling what happens in the veins if a tiny amount of this stuff got into your bloodstream is complicated because it destroys matter, which doesn't really happen in the real world. If my understanding of fluid dynamics is correct, though, it should look something like this. The oil is injected into the bloodstream. The heart pumps, pushing blood down the veins. A pipe system like the circulatory system works because each molecule is pushing the molecule in front of it, which causes the entire system to circulate. However, there isn't any way for the blood to move the oil, because as soon as the blood touches the oil, it disappears, so it can't impart energy/motion. Your blood won't be able to circulate at all, just get sent into the blob of oil and disappear.
On the other side, ahead of the blob of oil, the blood is getting sucked slightly backwards, trying to fill the vacuum created when the blood touches the oil. Long story short, your blood can't circulate. Also, floods are generally incompressible, so your blood can't expand to fill the vacuum, so you get little pockets of vacuum in your veins. Your blood vessels around the oil start popping, and you start bleeding internally, but that's cool because your blood isn't gonna stick around long. It's gonna start boiling off, unless you get a break in your blood vessels to let air in to fill in the gaps.
You would certainly be dead in under a minute, and the instant that oil touches your blood, you're a goner. This might not be 'instant death', but it's horrible and quick and irreversible, which is semantically good enough for me.