Syncing MS Project with SharePoint - Outline Level Field not Available

I am successfully synchronizing my MS Project Plan with SharePoint, however the Outline Level field is not available to map to SharePoint. I've tried creating a custom field with a formula that populates the Outline Level, however you can't mapped fields with formulas to the SharePoint synchronization.  I am ultimately trying to get the Outline Level in SharePoint so I can easily filter on Milestone summary tasks (i.e. Outline Level 2 and 3).

Any thoughts/suggestions/advice is greatly appreciated.

Best regards,

David