Wow. Straight 30 in winter. Talk about old school. Heck, if it was the correct old school, they should have recommended straight 20 grade. Go look at the Nebraska tractor tests from the '50s, '60s and '70s. If they were tested in winter, they all were using a straight 20 grade.
I use the T5 in my equipment and I learned to do that the hard way. I have a loader tractor I use in the winter. It HAS to start when needed. For a number of years, I was unable to plug it in because the barn didn't have electricity. I ran 15W40 in it. One cold winter, I couldn't get the thing to spin over fast enough to start. Cold battery, Cold, thick oil. No crankee fast enough. No startee. 10W30 made all the difference... and it wasn't even the T5 then, which has better CCS specs than the old Triple Protection I had been using. I run it year around in the loader tractor and the diesel pickup.
Definitely plug it in if using the straight 30 grade. Best even with the 10W30. I keep the loader tractor plugged in during winter but the cable is on a thermostat that kicks on at 35F. Much better for the engine to start warm and it gets you working faster too. If you have a cab and heater, gets you warmed up faster too!