The major reason to use DOT 5 fluids is the high temperature rating 500 deg F, however I doubt you are going in the V8 supercar series in a Mk3.
"If silicone is introduced into an older brake system, the silicone will latch unto the sludge generated by gradual component deterioration and create a gelatin like goop which will attract more crud and eventually plug up metering orifices or cause pistons to stick."
DOT3 and DOT4 will absorb moisture over time and therefore require fluid exchanges. Silicon fluid doesn't absorb water which may be an advantage until water gets in and then it will not absorb but remain as a globule sinking to the lowest point of the brake system rusting pistons etc.
Of interest "The only major user of silicone is the US Army. It has recently asked the SAE about a procedure for converting from silicon back to DOT 3-4."
from: http://importnut.net/brakefluid.htm
In referance to Diana's second paragraph I doubt there would any sludge as this truck sits now.
Thanks to Mick's running around...
Refurbished wheel cylinders.
NOS refurbished master cylinder/booster assey fitted(New ruber to booster, master striped,inspected and re-assembled).
New rubber lines X3 after blowing steel pipes out with compresed air.
New RHR steel pipe also(from "T" to wheel cylinder).
In addition to the above we cleaned and greased all the adjusters and expander units.
Brake shoes where hardly worn but some of the bell cranks needed freeing up.
