Passat 3B: Oil in Coolant - Causes & Fixes

I have a Passat that is pushing oil into the coolant.

After that, I replaced the oil cooler, and it was quiet for 250-300 kilometers.
Now, it's leaking oil into the coolant again.

Oil cooler again, or something else?

The original radiator had the part number 028 117 021L.
I installed one with the number 028 117 021B. It was from an automotive parts store.

According to the dealer, index B was changed to K and L. Were there any issues with that?

Edit: An AVB is not an AJM.

I'm going to bypass the water connections on the oil cooler and see what happens.