Some time soon I was hoping to change the brake fluid on my '90 200 (just cuz). Can anyone tell me which DOT to use and what are the advantages/disadvantages. Thanks.

One of the DOTs [4 or 5 ?] is silicone based , boils @ considerably higher temp, and is hydrophobic [won't absorb water]. It is also incompatible with the ester based fluid that's in there now so you must completely clean out and dry the system before installing it. Hopefully someone else will know the correct DOT #. Unless you are unhappy with what you are using now it's probably not worth all of the trouble as dirt bike brakes don't ever run hot enough to need it.

You can mix DOT 3, 4 and 5.1, but NOT DOT 5.0.

DOT 4 and 5.1 fluids must meet higher wet and dry boiling points than DOT 3 (wet is the boiling point when the fluid is contaminated w/water). I have boiled brakes before with old DOT 3 and it is a PITA. IMO, its worth it to spend a little extra and get Motul 600, it is a DOT 5.1 and has extremely high boiling points.