My new-to-me MiniMax jointer/planer came with the slot mortising attachment. I had occasion to use it this week, but am really wondering what the best tooling is. The instructions that came with the machine are silent on that point. What I tried was a four flute flat end milling bit I bought from McMaster. It would take only very shallow cuts (~1mm max), and at that there was vibration and a tendency of the bit to grab and run in the wood.

So, is an end mill the right tool, or am I better off with a router bit, for example a spiral upcut? The clearance for chips is much greater on the router bit than the mill.
Would I be better off with a two flute end mill? Or is there one made specifically for wood?
The two end mills I have have a flat on the shank, it that required to hold them in this machine? (I did manage to knock the bit out of the machine the first time I took too deep a cut, not sure whether it was correctly inserted or not, I couldn't tell by feel if something was engaging the flat or not)