It is not Ok to mix DOT5 silicon brake fluid with any other fluid.
If you are talking about Valvoline Synpower though, it's a standard, though high quality, DOT4, all of which are synthetic, and fine to mix with most any DOT3/4 with.
That said, why are you "topping off" brake fluid? If there is a leak, you need to fix it, and if it is the disk brakes wearing, you should not top off because when you change the brakes, you'll overflow the reservoir.