value in ms
&sync=0 will do the same thing -- it will try to auto-sync video and audio.
Tiny negative offsets may work, like
&sync=-25 is possible, but large negative offsets will not work.
No clue if it is supported in OBS or not; works in Chromium/Chrome v76 or newer tho. Useful if the video device has a large delay that needs compensating for that isn't fixed automatically.
&sync=500 without the
&buffer command also will only add an audio delay; there will be no additional video buffer or delay.