Magnum 18 - oil weights for cold weather

So, my next question concerning my new-to-me Deutz Allis with an M18S. It's set up with a plow, I'll be using it in cold weather. My manual says to only use SAE 30 over 32 degrees F, and is says do NOT (yep, in capital letters) use 10w30. But then, under the little graph with the degrees and arrows, it says that 5w20 or 5w30 synthetic can be used up to 40 degrees. I'm fine with using synthetic, and I like the idea of up to 40. There are days when you're plowing way below freezing, but also still plowing on days when it's warmed up and above 32 degrees. But it just seems strange to me - to go from straight 30w to 5w20 - wouldn't it make sense to go from 30 to 10w30 synth? I'm new to using these Magnum engines in the winter - I've used Commands before, and with their hydraulic lifters, it was pretty easy - 10w30 synth, summer and winter. I love my Kohler manuals, but I've found some pretty glaring misprints (or just plain old mistakes) concerning things like spark plug gap, gasket application, and torque specs. So I normally listen to manuals, but I'm a little nervous about going from SAE 30 to 5w30, especially in an engine with solid lifters and 500 hours on it. What do you all think? I'd love to hear some opinions, thanks in advance!

Magnum 18 - oil weights for cold weather