My Montana / LS tractor (Mitsubishi engine) states to use 10/30 Diesel oil; that is what I use. My manual has a temperature chart that you can use to pick the best oil grade depending on you ambient temperature (i.e. Florida vs Canadian temperatures).
This is from the Mitsubishi engine operators manual (not LS or Montana); the graphics pretty much explain it :laughing:
use a c rated oil that meets your temp / climate range.
much of the us is 15w40.. northern areas will be 10-30 and in some extreme hot areas I have seen 40w straight.. though it's not good for moderate climates due to wear on starting.. IE.. 15w40 being better..e tc.
Pretty much any of the diesel rated oils- I use Rotella-T 15w40 because I used it in fleet applications and it was great. There's also Castrol and Mobil Delvac and others. As long as they meet specifications they are all good, use a decent filter and change every 100 hours and they will all work fine.