What type of insulation should I use if my in-wall oven is heating the surrounding cabinets?
I would not recommend additional insulating wrap for the unit itself, this could cause it to overheat and malfunction... or even catch on fire. You should be able to find the model and serial #s on a placard inside the door.   Check the manufacturer's minimum clearance requirements, if the cabinets are too close then you have the wrong oven or improperly constructed cabinets.

You may be able to add a layer of 1/4" drywall (or some other insulating layer) to the inside of the cabinet compartments, this would slow down the heat radiating from the unit to the cabinets. But make sure manufacturer required minimum clearances are maintained.