2004 ktm 450exc hydraulic clutch

Mineral oil    

Hydraulic oil in a KTM clutch? Hmm, doesn't sound right.

The recommended fluid will be written on the master cylinder cap. If it's mineral and you run dot 5.1 brake fluid in it, likely you'll run into issues with the rubber and gaskets in the system. I think you'll have trouble with leaks, but others might have tried it? 

You might try a search for 'mineral oil brake alternative' ... you'll get some results. 

Likely you'll be able to find mineral oil at a bicycle shop. 

Don’t run brake fluid, it’s supposed to take mineral oil. I’ve used 5wt fork oil in my 05 200exc and klx110 with a magura clutch. No problems. 

That era KTM used mineral oil...lots of options including plain old mineral oil available at pharmacy. Better fluids though...like Rickyisms fork oil(5wt or lighter), shock fluid, ATF and so on. No real need to buy expensive mineral oil that may be a trek to find(maybe not). 

The older bikes don’t use brake fluid. 
As others have said, just use a light oil like fork fluid.