How to sync existing leads to SFDC without changing owner?

The auto assignment rule only triggers when the lead is synced for the 1st time. If the lead is already in sync and assigned to an owner it shoud stay the same.

You say in some cases it changes. Do you have any examples? Could their be a workflow rule in SFDC that triggers the reassignment?
 

Agree with Cathal, that's the only situation that could occur.

I ran a manual sync today on 400 leads and set the assignment to auto, about 40 leads changed from one rep to another

Hi Idan, do you have any triggers in SFDC that change the owner? Just like Cathal, I've never seen owner changes because of running "sync Lead to SFDC" on Leads that are already synced with SFDC. 

Also, there is often no need to call "sync Lead to SFDC" for existing Leads because changes will automatically be synced as part of the periodic sync. Often I put a "choice" in the "Sync Lead to SFDC" flow step that says "only if SFDC Type is empty", meaning that it will only run for Leads that are not yet synced to SFDC. 

If you still see issues, file a support ticket and provide the list of 40 leads that changed owner. If it's something specific to your SFDC, we may not be able to help, but if it's something that comes from Marketo, we can do more detailed troubleshooting. 

thanks for your replies. I'll check the assigning triggers in SFDC