Syslog is sourced from the management interface unless you want to change it.
If you do want to change it, you can go to Device > Setup > Services, click the Service Route Configuration link and the Select radio button on the dialog box. That gives you a list of all the management interface sourced functions that can be individually configured:
The box on the right lets you specify a source address as well for specific destinations, if needed.
Edit: I forgot your second question. There's only one management interface (though you can set up other interfaces so that they work to manage the firewall, the actual management interface is the one configured under Device > Setup > Management), so it will send traffic using that specific interface.
Hope this helps!
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