Do you "need" to change grade, or just "want" to?
1) what proof do you have that the current lube you are using (and presumably used last winter) did a poor job? Do you have UOAs and PCs to show the lube failed to protect in some manner?
2) what manner of measurement would indicate an success in this circumstance? What is your measuring stick that will be used to judge a meritorious approval?
While not common, there are examplese of 10w-30 dino HDEO doing quite well in today's modern diesel engines. The thinner grade will assist in a bit quicker flow, and proabaly a bit easier starting. But don't expect miracles overall. There won't be a big shift in performance or wear. The noise of typical operation in terms of wear is greater than what you'd be able to prove in singular season observations.
I see no "need" for T6, unless you intend to greatly increase your OCI to some very long factor.
There is a difference between feeling and knowning. Unless you have quantifiable performance parameters that you'll use as a baseline, then you'll never be aware of any improvement, or lack thereof. Once you establish the baseline, then what measurables would indicate to you that success has been achieved?
Don't look at inputs; quit looking at the VOAs. Most any lube is "good enough" for what you're to put it through. Rather, look at UOAs and other tangible criteria; what indicators show a failure of 15w-40 where 10w-30 0r 5w-40 would usurp it?