I know where the brake fluid and windshield wiper fluid is kept, but what is the "mineral oil" reservoir for?

 

Clutch fluid reservoir, same fluid as power steering, since the turbo uses a hydraulic assist for the clutch system.

 

Ok, thanks. And why is it "mineral oil?" I would have thought some kind of hydraulic oil.

 

It is a synthetic hydraulic fluid, http://www.autobarn.net/ch252-0008.html That is what used to be used, but was changed to the new Pentosin, which is basically the same stuff. Used in Power steering, Clutch (turbos only), and Cab top operation. Amazon.com also sells it for same price.