Define calculation fields as:
List ( OEMpart ; MFGpart )
Use them as your match fields.
This is useful as a Look Up relationship, but it would be best to set your own internally generated ID for each item in inventory.
it says (missing index)
At least one of these must be an indexed field. The calculation field defined as List ( OEMpart ; MFGpart ) must be defined in the same table as your OEMpart and MFGpart fields.