Dot 3, 4, and 5.1 are all ester based fluids, just with different boiling points. Dot 5 is silicone based. If there is any residual ester fluid in your system when you add silicone base, the silicone will gel. Also, I believe dot5 will have some sort of negative impact on the seals and rubber components of an ester system. May be wrong on that last part and it's just a continuation of the gelling problem, but still, just say no to dot5.