It would be REALLY helpful to know just where in the "usa" you live ...
15w-40 in the southern states is no issue, even in winter.
15w-40 in the upper plains is a big deal and will result in cold starting difficulties without aid of strong batteries and/or a block heater. Here, the thinner grade would be a big plus.
There is excellent service to be found in the 10w-30 HDEOs; they are capable in nearly every temp and for every service factor. I suspect your local JD folks are pre-programmed for the "thicker is better" mentality; predominant in the diesel world. But I and several others have had excellent success with thinner grades and have the data to back it up.
The "need" for syns is overblown. I can only agree to two conditions that would necessitate such use:
1) uber-stupid-cold temps (-20 F or below)
2) really LONG OCIs
If you're not going to be involved in those operational events, then syns will end up providing no benefit for the extra cost.
While I like to recommend dino 10w-30, it's not often easily found for all. JD has an excellent 10w-30 dino; roadrunner1 has many a testament for it's effectiveness. Given the presumptive small size of the sump, I realize that nothing is probably going to break the bank as far as cost goes. I would recommend T5 in 10w-30; it is an excellent blend of performance, ease of availability, and cost. Typically $16/gallon at about any WallyWorld.