Delo 10W40 is a diesel grade oil, I would not bother with switching to a lower weight oil or synthetic for a few days in the cold. The truck may start a little slow, and might want to give it 2-5 mins to and warm up a little but it doesnt warrant changing your whole oil regime. Also as the tech says, Diesel oil is a special grade, you willl end up doing more damage to your motor by running a non diesel grade oil in it vs an oil that is viscuss.