For my winter vehicle (1997 Bonneville). I use a block heater whenever the temperature falls below -10C (15F) over night which here is pretty much late December to early March. The car starts much easier, oil pressure rises right away, you get heat sooner, the ECM adds less fuel since the engine starts warmer, what's not to like. I also use a timer and have it come on a little before 5:00AM.
It was -32C (-24 F) 2 days ago. At home the car started great, after work sitting all by itself the starter turned it over way slower. It was so cold the seat foam was like a rock until it sucked some heat out of my butt and it warmed up.
A block heater does a better job than an oil heater since there is so much more mass. Heat the oil up all you want it won't be the same as heating up the coolant. I have heard of inline pump style heater that go in a heater hose and circulate coolant around. Its much easier to install a block heater.
For fun on a clear day last winter I took the Buick out at -20C. Its like not having a turbo the spool was instantaneous. The wheels spun instantly on the frozen pavement.
