I believe for my "lavalab" level I just used "!fluid2" which is in one of the standard WAD files -- because that was my first level, long before I even considered using non-"standard" textures. I don't actually have my RMF file handy, but I'm pretty sure it was "!fluid2".
And then, to make it look good, I added a suitable light colour to that texture in my RAD file...
Admittedly it still looks a little too fluid to be lava, but it's pretty effective...