Regarding LCL in Neon Genesis Evangelion, my conclusion was that it was Lilith's blood.

Evidence (Warning, SPOILERS):

Episode 7: Shinji Ikari: "Keeping the Earth safe. That's what Eva's for. Come to think of it just, what is Eva? This entry plug smells a lot like, blood. But I feel so comfortable and relaxed here. Now that I think of it, I don't know anything about it."

Episode 15: Kaji and Misato stand in Terminal Dogma outside a (rather imposing) door marked "LCL Plant".
Kaji: "...But you should know that the Commander and Ritsuko are keeping something from you, too. And this is it!"
(He swipes his ID card and the door opens, revealing a crucified giant that also happens to be missing its legs and lower torso. From these wounds flows a yellow ichor, filling the huge chamber.)

Episode 16: Shinji, trapped inside the 12th Angel, says: "The water’s getting cloudy! The purification system is breaking down! It smells in here! Blood! This place smells like blood!"