Lead owners are required when syncing a lead to Salesforce, but is there a way we can sync them without assigning a lead owner? Yes there is, and it's called lead queues!
Solution
How to do it:
Lead queues must be set up in Salesforce first. Like lead owners, the queues must be visible to the Marketo Sync user before they can be selected.
Select the lead queue you want leads to be assigned to in the 'Sync Lead to SFDC' or 'Change Lead Owner' flow steps.
Qualified leads that pass through this flow step with then be synced to Salesforce and assigned to the lead queue rather than a lead owner.
A few important notes:
Only leads can be assigned to a lead queue, contacts cannot and must be assigned an owner.
Assignment rules in Salesforce can conflict with the request sent from Marketo. Double check these assignment rules to make sure there are no conflicts.