I wouldn't run 15w-40 in the winter; but in the summer it would be good. My choice would be 15w-40 Chevron Delo.
Otherwise, for year round oil, change that Trooper over to Rotella synthetic 5w-40, this oil is labeled a synthetic, but is really an ultra purified regular oil suitable for diesel/gasoline engines with heavy detergents.
Personally, I have to wonder if you started using oil because of switching brands, BUT that doesn't mean it'll last forever, it may taper off. Then again, it's possible that that engine doesn't like Mobil 1 - or, maybe it likes it TOO much
I have a truck that's had a life of Havoline, with the exception of a few variables, one of them being Castrol GTX about 6yrs ago when I only had 60-some thousand miles. I remember during that 3k miles that my truck used almost 1 qt, I know that doesn't sound like a lot to you, but on a truck that still doesn't use any oil in 3k miles, that was a lot.
I've never used a 15w-40 in my Isuzu engine, and I'm not sure how much that would effect performance, but it can't hurt to try. Just wait til the night temps are above 30F personally, that's a pretty thick base to get to the top of the engine on a cold morning.