Wolf -- first of all "mineral oil" in European terminology is NOT the same as USA. It means mineral-based hydraulic oil, not the stuff you get at the drugstore. Fork oil is hydraulic oil, so is ATF, or you can get the type (AW 32 I think it's called?) that they use in tractors. You can get Magura hydraulic oil from KTM, but it is VERY expensive. I've used 2-wt fork oil, and liked it the best, but recently my master cylinder started to leak. Replaced the plunger assembly, now up to 10-wt fork oil, but it's still leaking. Ugh. Hope the mster cylinder isn't scored. And by the way, speaking of scored master cylinders, you've probably learned by now to avoid the Enduro Engineering (and other brands) of easy-pull clutch levers. They have a near-100% failure rate and can permanently damage the master cylinder. I love Enduro Engineering stuff, but any type of easy-pull lever puts the wrong angle of attack on the plunger.
Hope this helps.
SlowJack