Was checking the racelist last night and set the filter to GTR, the list only showed servers which had all the GTR cars enabled (and no others)

Yet it did not display a server which was only running one of the GTR class cars (and no others)

Would be nice if the class filter would also detect whether a server was running any of the cars in the class (and no cars from another class of course)

You can check this by clicking the GTR car buttons individually. If you press the "GTR"-filter LFS thinks that you want to find a server with all the GTRs enabled.

Personally I don't use the predefined filters at all. I just "download" the full list and then see what's going on, by clicking individual cars

You can list servers by selecting cars. (want to be allowed on server, or not)
Even a XFG + FZ5 XRR combo is possible, just click on the cars. There are 3 colors... "normal" which is like "don't care", red "don't want to allow" and blue "i want this to be allowed".
So all of the combos of car selection is can be set

Try it and have fun with custom filters
*EDIT: 4 just the GTRs - GTRs normal, others red, and there u go

I think SKurjz means different thing: show not only GTR servers, but other servers where only GTR cars in race too. This would be great option and I thought about this long time ago, but problem is (in my opinion) checking every server for cars in race take looong time