i'm not sure what is meant by "synthetic" vs non with regards to brake fluid
however when it comes to substituting with modern products this is the rule
Dot 3, 4 or 5.1 brake fluid are all compatible and can be mixed to any ratio.
You will simply end up with a mix that is part way between the specifications of the two fluids.
example 1, vehicle states to use DOT 3, use any of the above, or a mix.
example 2, vehicle states to use DOT 4, use DOT 4 or DOT 5.1 or any mix thereof.
DOT 5 is silicone based, it CANNOT be mixed with the above, must use only DOT 5 for replacement fluid.
example 3, vehicle states to use DOT 5, only silicone type maybe used
