<VV> dot 3,5

[cfm]

Some of the messages lately are about brakes and brake fluid.
Hypothetically, if you just bought a car, or have owned one for a while and find yourself determining what kind of brake fluid to add to your system (if required), how is one to know which fluid to use?
Do you default to DOT3?
Can you mix 3 and 5?
Can one inadvertently add a silicone based fluid to a non silicon system and get an adverse reaction?
What type was OEM from Chevrolet at the factory?
cfm