Not going to comment on the oil to use, but as a person who lives in a cold climate. We can easily get down to -50 here in winter, but even at -10, -20, -30, the block heater does almost nothing to warm your engine oil.
Crawl under a vehicle on a cold morning, block heater has been plugged in for 12 hours, and put your hand on the bottom of the oil pan.
Its basically the same temperature as it would be without any block heater. So while you have warmed the coolant some, it doesn't help warm the oil to make it pump quicker on initial start up.
If concerned about it, oil pan heaters are a common item here. My F150, girlfriend's Ram 1500, and my Western Star logging truck, all have oil pan heaters, along with the block heater. Warming up the oil makes a noticeably larger difference on a cold climate morning start, than does the block heater. Thankfully I have both, but if forced to choose between them, I would keep the oil heater.
