Your mechanic is very mis-informed! Brake fluid will not catch fire. It will smoke alot when poured on to a fire.
The o-rings in the system will not be harmed with the Dot4 fluid. How many guys here who have changed brake fluid here with no problems. Alot! If you just use the Z as a DD, and don't track it. Just bleed the brakes once a year, and you will be fine.
