With a high quality synthetic, I find the need for a block heater is much lower in -20 to -30. I say this as some who parks on-street.
What is absolute kryptonite for my car in these temps is the battery going flat. As such a battery warmer has far more of an impact on the car starting for me than a block heater. Admittedly, part of this is because of a power hungry Compustar remote starter, and the fact I may not drive the car for 3-4 days.
This is what I bought, and it seems to extend battery life noticably:
https://www.canadiantire.ca/en/pdp/t...w.ds#store=930
My 85' Volvo came with one of these from the factory - I always wondered why it wasn' a more common thing.