All engine instruments need to be in the green prior to departure. On cooler days – or on the first flight of a day – in can take several minutes all the way into the runup phase before the oil temperature is in the green.
With the Twin Star, insufficient oil temperature can also lead to the ECU TEST buttons having no effect during runup.
Please be patient, and wait that extra minute or so before proceeding with the runup so that you have a safe flight – treat any aircraft engine like your life depends on it!