In some cases you may want to make your SFDC field read only and not allow Marketo to write to the field. Making a SFDC field read only may be important if your SFDC instance is connected to a ticketing or some other system which has address or other information which should never be changed.
If you make a field read-only in SFDC, the value of the field in SFDC will always prevail. To help prevent data discrepancies you should not use the field on any forms or any Change Data Value flow steps. You should also block updates in Marketo from all input sources. If the SFDC field gets populated in Marketo first you may see the following sync error: INVALID_FIELD_FOR_INSERT_UPDATE: Unable to create/update fields: [name of field]. Please check the security settings of this field and verify that it is read/write for your profile or permission set.\n
Eventually when the field is populated by a SFDC user the new value will sync from SFDC eliminating the discrepancy and resolving the sync error.
In Marketo if you don't have Block Field Updates set for "Flow change data value" you can temporarily change the value of the field in Marketo. It will revert to the SFDC value the next time any update is made to the record and data is synced between the two systems.
SFDC Account fields are automatically set in Marketo as read only. For SFDC Contacts in Marketo you cannot manually edit Account fields in Marketo’s Lead Detail screen, while you can use a Flow Action to change an Account field value. The change will be temporary until the Account is updated. The value changed will revert to the SFDC Account value the next time any update is made to the record and data is synced between the two systems.
Read only fields in Marketo.
Here are some additional resources for learning about the Marketo integration with SFDC.