Justin,
If you want to run a diesel oil in gas engines designed for 5w-30 or 10w-30 grades, I'd recommend using a 5w-30 or 5w-40 grade year round. Some of the ones I'd recommend are listed below:
Delvac 1, 5w-40
Amsoil Series 3000, 5w-30
Castrol Elixion, 5w-30
Delo 400, 5w-40
Rotella T, 5w-40
Petro Canada "Duron", 5w-40
Valvoline Premium Blue "Extreme", 5w-40
There are also some petroleum diesel oils available in a 10w-30 grade, like Delo 400 and Rotella T. I think these would be preferrable to a 15w-40, particularly for use in cold climates. You could probably go with the 15w-40 in ambient temps above 40F and it would work okay.
TooSlick
Dixie Synthetics