brake fluid & DOT rating

[Mark McDonald <tomswift@xxxxxxxxxxxxx>]

Okay, here's another one that will likely get 20 different answers, all 
contradictory.

For the first time I'm starting to wonder about DOT ratings and what I 
should put in my Chryslers.  I hate to admit to such ignorance, but it 
never really concerned me before.

I was reading an article which stated that DOT 5 should not be used in 
an old car.  Correct?  (DOT 5 is silicone brake fluid, right?)  More 
important, you CANNOT mix DOT 4 and DOT 5 because they are not 
compatible, will attack each other, and the seals.  Yes?  No?

However, DOT 3 and 4 are okay for old cars and CAN be mixed.

DOT 2 is strictly for older systems-- drum only-- and should probably 
be thrown out.  Right?

How often should people change the brake fluid in their, say, 1968 
Imperial???

Thanks in advance y'all
MM