Hi @emilyk8r 
 

If you select leadtime auto enabled in supplier for purchase part record then the lead time in Inventory part will be automatically updated.

No need to keep lead time always as 0 , Suppose you have existing lead time in inventory part record, but if you enable leadtime auto , then when you change the supplier manufacturing lead time , Inspection lead time system will automatically calculate purchase lead time and that will be fetched in inventory part record overriding the existing value.

Field description for the Leadtime auto says:-

If this option is enabled, lead times will be calculated automatically when the actual receipt date is later than the** Promised Delivery Date** field for the purchase order line. This setting should be used with caution as every time a late delivery is received, the Supplier Manufacturing Lead Time field in Supplier for Purchase Part page and Purchasing Lead Time field in Inventory Part/Main tab, Lead Times And Supply Dates section will be recalculated automatically. The Purchasing Lead Time field in Inventory Part/Main tab is seen as the standard lead time for the part; used in various places in the application. Therefore, having this option enabled may result in an unpredictable standard lead time for the part.for Purchase Part page and Purchasing Lead Time field in Inventory Part/Main tab, Lead Times And Supply Dates section will be recalculated automatically. The Purchasing Lead Time field in Inventory Part/Main tab is seen as the standard lead time for the part; used in various places in the application. Therefore, having this option enabled may result in an unpredictable standard lead time for the part.