Somebody I know (who is not me, he just looks like me and drives the same cars and has the same fingerprints and stuff) once used gear oil in the clutch hydraulics to shore up a failing master cylinder. It worked fine, in the summer time.
Backstory: A friend of mine (uh, I mean his) told me (him) about a friend of his that did that when the master in his REPU started to die, and it lasted until he got rid of the truck.
So there I was, about a year later, and 1/2 hour away from going to the dragstrip, when the clutch master in my car decided to die. No pass go, no collect $200, just flat out dead. So, the guy I knew completely flushed out the system using a vacuum bleeder and compressed air, and poured in some 75W90. Would would it hurt to try? The master was already dead, and the slave was under warranty.
Air bubbles move SLOWLY through gear oil. Spare master cap, 7/16" drill bit, tire valve, and a bike air shock pump, and I had a pressure bleeder. Bled the system out in about 5 minutes. Pushed the pedal down. It came back up! Pushed the pedal down and staretd the engine. The clutch would disengage! SUCCESS!
Well, success in the summertime anyway. This friend of mine, he lived in California. It doesn't get much cold, there. It gets cold here. When it got below about 45 degrees, the clutch action got a little "thick". When it got below 30 degrees, you could push the pedal down and count to three before it would come back up enough to let the clutch engage. Took forever to warm up, too.
But, sure enough, it let that dead master cylinder limp along for another two years before it started bypassing again. Swapped in a new master and slave and used brake fluid like you're supposed to..