I get -30C (with coolant block heater) to +30C starting temp range here in S Manitoba and the first few changes used the recommended 10w-30 Kubota oil, loath then as yet to use anything not explicitly recommended in the new tractor's maintenance manual.
I had read here of T6 5W-40 doing well in these engines. It turns out we had an old VW TDi (long since retired) that had thrived on t6 5w-40 and I had exactly 4L left in a 19L pail sitting there in the garage melee. In it went at the B2650s 400 hour service.
I certainly noticed "easier" cold starts, whether biting cold or around freezing, in that it quieted faster from the initial always present diesel rattle bang cold start noises. Idle smooths out and quiets faster.
And honestly the power and responsiveness using this oil compared to the Kubota 10-30 semi-synth seemed better.
Makes sense to me, with the lower start viscosity 5W vs 10W, and higher run temp viscosity 40W vs 30W.
"Coverage" is enhanced at both ends?
I'm hoping to hear from more knowledgeable on this, as oil can be a hot topic here and it's interesting to hear more from those better versed.
But the T6-5w-40 does a better job here in both heat and cold than the 10w-30. And I wouldn't go near a 15W- as starting temps don't go above freezing here in the morning for 6 months of the year.