Hi @hoaphan199 , it is unclear if the custom sync filter can be employed to restrict Opportunities from sync in Salesforce-Marketo integrations, as it can for Dynamics-Marketo integrations. (Related: for some reason the sync filter is not even documented for Salesforce integrations in the Marketo Product Docs. ??) If you are interested in using a field to control the sync of Opp records from SFDC to Marketo, ask Marketo Support if it is possible for the Opportunity object. We have seen it used on the Account object.
We have had the experience that all default objects (Lead, Contact, Opportunity, Account) were required to be visible to the Marketo Sync user in order to enable the integration without an error message. But, adjusting the permissions for Opportunities is worth a try. You could set permissions at the Opportunity field level as well, to hide the fields from the sync.