A little Google-Fu, and some bookmarks I keep on this subject, and here's what you need to know.
For starters, your minimum temps are still well above freezing and in your case 40 being the lowest. You can set your lowest temp to 32. You'll see why in my links.
Next, your highest temps are going to be around 100 or so. You can set your highest temp to be 104, and again see the links as to why.
Using these temps makes it easy to figure out you have some choices in oil weights, and by referencing the charts in the links below, you could use:
Links for more info:
Really what you need to look for at this point is what standards and specs the oil Can Am recommends you to use ... other than their stuff, of course.
So the specs you need to look for are clearly on their labels:
API SM (API rating "S" is for spark plug engines; "C" is for compression or diesel engines)
JASO MA2 (MA2 rating is for engines with catalytic converters)
This means you need to find an oil in that weight range that uses either the API SM and/or JASO MA2 specs, and a full synthetic would be best due to it's ability to protect the engine, and the additives and detergents to keep the gunk out.
I've been using Mobil 1 full synthetic 5w40 diesel oil, and no issues with it.
The other thing you mentioned was dirty oil. Oil doesn't really get "dirty", per se, but if you're concerned about it then I would recommend using Blackstone Labs for oil analysis:
The kit is free, but it's $25 to get their report after sending in a sample. I've used it for my diesel truck and on the next oil change for my Commander I plan on doing it since Mobil 1 is a known oil to Blackstone (unlike the Can Am brand) so they already have industry data to use to compare to my sample. They can tell you oil life (change it sooner/go longer), if there is engine wear or not, and all kinds of other data you can compare against previous samples so you truly know what's going on.
Since you're having "dirty" oil, I would recommend switching to Mr RPM's zero port oil filter adapter and use a Baldwin filter. Baldwin filters are what I use due to their better filtration for my diesel, and they include one with the oil adapter kit.