OK, this ought to clarify the question about which oil to use in very cold (but not Arctic-cold) weather. Here is an authoritative reply from a guy who knows what he's talking about. I'd quote him, but I didn't get his permission:
I'd suggest using the Rotella T until you get at least 10,000 miles on the engine, then switch to the oil of your choice. If you install synthetic oil too early, the engine will never get broken in. If you experience very cold weather, perhaps go to a diesel rated 10W-40 petroleum based oil for awhile to aid break in as well as cold weather starting. Be sure it has an API (the donut symbol on the container) of CH-4/SJ or better. The 5W-30 synthetic is only for temps consistently below 32 degrees F or less, so it is not for
warmer weather operation.
Best,
Jeremiah Joiner
I'd suggest using the Rotella T until you get at least 10,000 miles on the engine, then switch to the oil of your choice. If you install synthetic oil too early, the engine will never get broken in. If you experience very cold weather, perhaps go to a diesel rated 10W-40 petroleum based oil for awhile to aid break in as well as cold weather starting. Be sure it has an API (the donut symbol on the container) of CH-4/SJ or better. The 5W-30 synthetic is only for temps consistently below 32 degrees F or less, so it is not for
warmer weather operation.
Best,
Jeremiah Joiner