Any 5w-30 or 5w-40 HDEO will work fine where you are at.
But I would suggest you consider the "need" for syn's at all ...
I looked at the average weather data for your area in NOAAs database. I reviewed 10 years worth of data, looking at the two coldest months each year. Here is the weather.com review of your area:
http://www.weather.com/weather/wxclimatology/monthly/graph/USMT0188
While not detailed like the NOAA database I reviewed, it is an independent confirmation that my summation is correct.
Here is how your weather stacks up in Kalispell:
1) Your average low is +16 degF for Dec and Jan.
2) The standard deviation of data shows that you will see variance in 7 degree nodes, approximately.
3) Your area will only see temps below -10 once a year, as an avearge (some years more, some years less).
4) You will RARELY ever see anthing below -20F (as defined as once in only a few years or more).
Frankly, while it does get cold where you are at, it does not get so cold that syns are a necessity.
Believe it or not, that data shows that where you are at in Montanna (southwest of Glacier) is only two degrees colder on average than where I am an in central Indiana (my average low is +18 degF). We actually saw -34 degF here several years ago, but in no way is that "normal" for us. And it's not "normal" for you either. I survive just fine with conventional (dino) 10w-30 HDEO in my Duramax and my Kubota L3430GST, and I drive both frequently in winter. Never had a problem with cold starts; never had a problem with wear as verified by UOAs. In your area, which is VERY much similar to my area, a conventional 10w-30 HDEO would do the job more than sufficiently.
I suspect you believe you "need" synthetics, and I hope I've presented evidence that will persuade you to reconsider what is "needed" versus what is "wanted".
But, if you absolutely must have something "better", I would suggest you consider the Shell T5 in 10w-30. FYI, the T5 in 10w-30 has the same cold cranking rating as does the full syn T6 5w-40; both are well more than what you "need" for Kalispell and your surrounding areas. Depending upon your OCI plan, the T5 would do an excellent job, and is readily available through the Walmart in Kalispell, or in Polson should you run down that far. Recently it was on sale for $12.99 a gallon. You'll find it nearly impossible to beat that performance, for that price, with that kind of availability with any other product.
You can use some rare and expensive 5w-30 HDEO if you
want to, but you don't
need to.
