A data point on (successfully!) using RUCs on a DL-marketed, AM-operated flight LAX-MEX.

Purchased a DL 8156 through Delta, though operated by AM 645. 006 ticket stock. PNR contained 3 pax. I am PM and two parents are nothing.

Since RUCs and Medallion complimentary upgrades are only available for member + 1 companion, I had to split off one of the 3 pax on the PNR.

I split off my dad into his own PNR, and upgraded his ticket with miles (sort of ridiculous at 25k miles a piece, O class needs to be available). At that time, there was only one seat available. For mileage upgrades, AM O class needs to be available and the ticket is re-issued into O class as an AM flight number (no longer DL) on 006 ticket stock.

A few days later, more O space opened up, so I used 2 RUCs for myself and my mother -- ticket was reissued as AM flight number, 006 ticket stock, into O class for both of us. This was an arduous process, probably took about 4 calls to process each set of upgrades due to agents not knowing what to do, or being mis-informed on how to search for space and telling me there was no space.

Not sure if my mom and I would have cleared the complimentary upgrade, but didn't want to chance it since we don't get a lot of holidays together, and I was okay burning RUCs for a 4.5hr flight.

A few pointers:

• If you can see an award ticket for the AM flight through Delta.com / the app / ExpertFlyer, you should be good to go for a mileage upgrade or RUC upgrade -- O class is the class you are looking for, not OY or RY --when calling, ask the agent to search for O class with the AM flight number, not the DL-flight number as this will generally be unsuccessful
• You cannot waitlist RUCs on AM -- you will have to continually call back for agents to check space, or you can setup an ExpertFlyer alert
• If you can see the space but the agent is claiming they cannot, HUACA
• For both mileage and RUC upgrades, you most definitely will need to follow-up on the mileage credit, as AM O class does not earn credit with DL
• For mileage credit, even though I originally purchased as DL-flight (and should earn under DL rules), an agent reconciling the mileage credit will most likely change the credit method to "distance-based" which can sometimes work out in your favor, depending on how expensive your fare was / status level

Overall, not a great experience, but in the end was successful.