Depending on the year of the car, your block heater will only turn on after a certain temperature (check the manual or block heater cord if it has a tag on it). That's will minimizing energy usage and just turning the heater on when it's needed.

Since your using synthetic oil you can probably leave the block heater unplugged until it gets down to around -20C. I have synthetic oil in my work truck (gas) and only plug it in when it gets down closer to -30C. You may have to play with it a bit.

Other options are to get a timer that just starts the block heater a few hours before you leave. Even better you can get a smarthome plug that you can program to turn on when the temp reaches a certain temperature or time.