8914 is 3M's Optically clear brand.
8518 has a little bit of a yellow tint to it - Not noticeable for decals / graphics, but when you put it on window perf it's pretty noticeable as it prevents the light from getting through, and makes it not as clear. In some cases, it pretty much makes it impossible to see through.
I mean, all overlam is clear... It's just there to protect the graphics, it HAS to be clear. Optically clear is meant for window perf, transparent window decals, etc.
I don't know why you don't think it's available in the US either...
Grimco | 3M™ 8914 Scotchcal Optically Clear Overlaminate
Request a sample from your supplier, and next time you do window perf... print an extra piece, laminate it with optically clear, then put them both up on the window and you'll be able to see the difference. Avery also makes an optically clear... Dol 4000 or something. We don't do too much window perf anymore, so I'm not familiar with the avery version. We use the 8914 quite a bit though, goes good with the optically clear printable 8150.