Your engine will be completely happy with 15W40 oil. It'll be OK with 30W (OE30) as well. It was designed to use OE30, but the 15W40 diesel rated oils in the LO are better in every respect and are much nicer to the starter motor and offer better lubricity in cold weather.
The LOs for my diesel generators tell the same story - designed around OE30, LO changed to multigrade some time in the 1980s or early 1990s.
The only reason I would be hesitant to run 15W40 in a gasoline engine would be because it did not carry an S* rating (SE, SF, SG, etc), which indicates suitability for spark ignition (generally gasoline) engines. This generally has to do with the pH buffer package in the oil.
For the record, I run 15W40 (Rotella T in my case) in my vintage motorcycle all the time and in my lawn mowers when that's what I have a fractional container of at change time. No fuss, no problems, no worries.
If you really feel the need to not run 15W40, then run 10W30 all the time - there's no advantage to seasonal changes to HD30. Be careful to stay away from "Energy Conserving" oils, though, as they may not have adequate shear lubricity to deal with the flat (non-roller) lifters on your 4A032.
