What oil?

[v8volvo]

Originally Posted by DieselScout

I used to use 5w-30 or 10w-40 in the Winter and 15w-40 in the Summer.
Now I just use Rotella 5w-40 synthetic, year-round.

^^^ This (or something like it) is the best plan of all.

"summer" and "winter" oils are really a thing of the past now that modern synthetics with wide viscosity spreads are commonplace and highly affordable. Not only does a synthetic 5W40 or 0W40 protect the engine better in both hot AND cold conditions than any grade of conventional oil would, it saves you having to think about the temperatures when choosing your oil, change oil prematurely to prepare for a different season, etc. There is no advantage, not even cost, in running conventional oils that require that kind of seasonal planning and also need more frequent drain intervals, etc.

Just picking a good oil in the right weight that covers all conditions well is the way to go. 5W40 is great for all but the most extreme cold situations. 0W40 only makes a difference when the temperatures are in the deep negatives. I run 0W40 Rotella T6 myself year round to assist with starting on the couple mornings per year in my area when temps reach -40F or lower, but even for that situation a good synthetic 5W40 would probably perform well enough. A 10W40 or 15W40 would be hopeless on the other hand.