As your brake pads wear, the pistons in the caliper move farther out, more fluid moves in to fill the void, and the brake fluid level drops. If you had a leak you'd know right away, trust me. So, that's where the brake fluid goes when the level in the reservoir drops.
All brake fluid is synthetic, just use dot 3/4 fluid, it'll mix with the existing. It wouldn't hurt to pull all the fluid out of the brake reservoir and replace it with new.
PS: Make sure that when you change the brake pads (when you eventually do), that you pull all that extra fluid out of the reservoir before you compress the calipers to install the new pads, all that fluid will move back into the topped off/full reservoir and pour all over the place.
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