DOT 5 uses a different chemistry. It is not compatible with DOT 3, 4, or 5.1 (just to confuse things). The two types of fluid don't mix, may require different kinds of seals, etc. Putting DOT 5 in anything other than a brand new system designed for DOT 5 will cause you no end of expensive grief.
My 2006 has abs plus the dreaded servo brakes, I would love to remove the servo but can't without removing the abs so I'm in two minds, it all works fine atm but when the servo goes you could be flying somewhere and have to stop with no abs and what BMW call 'residual' braking which equates to around 10% of actually what should be on offer..scary. This obviously won't happen if I take the system out and I'll be left with very very good brakes but no abs.