Alternatives for mineral brake fluid / oil (in a pinch)
The rubber seals in mineral oil levers and calipers are very easily damaged by using the wrong fluid, and should this occur replacement seals aren't typically available. Experimenting with alternative or in-a-pinch brake fluids is a bad idea for this reason. The results are unpredictable. In the worst case, a brake failure could occur.

Running a different viscosity than intended can also alter brake performance. In particular, higher viscosity fluids have been known to sometimes cause either slower or incomplete piston return. Of course the specifics of the hose, brake, piston count, pad spring, etc are all going to play into that, a fact that no meathead MTB forum poster is ever going to consider for a hot second before telling you to go put LMH in your brakes just because they did it and it didn't blow up.

There's a type of DIYer who loves talking about how they've had their bike set up with hack X for Y years and it's totally fine, so it of course will always be fine for everyone forever. That's a good type of person to categorically ignore. Most of the time, their shadetree nonsense doesn't work as well as they want to believe, and they're really only out to appear as though they have something meaningful to contribute, when in fact the opposite is almost always true.