Is it ok to top off reqular brake fluid with synthetic?

I believe all brake fluid is synthetic. Someone else can probably confirm that.

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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.

Thanks for the feedback folks. 10-4 vader

It's good to top off low fluid. Sucking it out first is normal and good procedure when replacing pads.
But adding fresh fluid keeps air and thus moisture down in the system, and mixes fresh fluid with the old. It's a win win situation.
And all brake fluid is synthetic, like mentioned. Just watch out for the silicone based stuff.