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.
'96 Impala SS
'07 Focus ST
'07 Burgman 650 Executive
'08 Gold Wing