Generally no problem, but let's not mix up the whole DOT rating with the "silicone" part of the fluid formulation.
Take a look at the attached files, this is a pretty good write-up that addresses both.
My takeaway is captured in the following quote (from the article):
...So why is silicone-based DOT 5 fluid more compressible than other fluids?
On their own, silicone-based DOT 5 fluids are entirely different animals than DOT 3 and DOT 4 fluids. In addition to having characteristically higher dry and wet boiling points, they also tend to have much, much lower viscosities. In other words, they flow more easily relative to temperature. One side effect of this chemistry is that there is more “room” for air to fit in-between the individual molecules of brake fluid than in DOT 3 or DOT 4 fluids. Note that we are not talking about big bubbles of air here which are visible to the naked eye, but rather microscopic amounts of air which are finely dispersed (entrained) in
the brake fluid matrix.
Now, all fluids have a certain amount of compressibility to start with, but adding even the smallest amount of air into the solution can dramatically increase the amount of elasticity in the system. In the case of silicone- based fluids, air is quite happy to take up residence between the brake fluid molecules, and as a result the fluid compressibility goes down. This is felt at your foot like stepping on a big spring. As you can imagine, more air = more spring...
Some pick up on that feel and do NOT like it, others do not, or if they do they do not object and are fine with the difference.
