All that matters is that they are the EXACT same size and made of a material that doesn't break down when it comes in contact with gas/hydrocarbons or oil.
I have benn running them for a few weeks now, 0 leakage...and I even used the smaller size on the drivers side injectors for an experiment.
The dealer is raping you..how more simple can you get than an o-ring?? You put them side by side, measure them, strech them, etc. They are identical. I may even say they are better especially the 7mm because the OEM ones seem to loosen and get weak over a short time. I have an 03 and my stockers were floppy which is why they tore when I reinstalled them. I'm guessing this is why so many people tear them too.
Note: The 7mmx2mm o-rings size difference is so close to the 8mmx2mm you can barely see it. Once the o-ring is on the injector the seal area is exactaly the same size. The only real difference is that there is less play when it is on the injector. This helps prevent tearing when installing because it stays alligned during insertion, much better than the looser 8mm.