I bought it from an auto-parts store, they looked it up through their computer and sold it to me...I later realized that they just go on the WIX website and see what it says. I did that and noticed that WIX offers two filters for the IS, one for IS250 and one for the AWD version. It makes little sense to me since the engine is exactly the same. Before I did the oil change, I wanted to make sure I had the right filter so that I wouldn't be screwed to find out that it's the wrong one. So I bought one from Lexus, and realized that they're completely different in size. I don't know if you can interchange them but I didn't, I just used the OEM filter and saved the WIX one for the RX350 that we have.
Also, when I'm gonna need another filter, I'll try to find out if the other one WIX offer is the same as the OEM one. If you look on their site it's #57173. From the picture, it looks the same as OEM, in terms of size.