Should be warm enough for 15w-40, guessing the 10w-30 is for the colder areas. I ran conventional 15w-40 in my 96.
I saw some 5 gal on line of the Mobil 15w-40, but only if you select qty 1.
Exact Fitfor your1996 Dodge Ram 2500
Mobil Delvac 1300 Super 15W-40 1Q; Helps extend diesel engine life for the most severe on- and off-highway applications; 10.4 QT Capacity; Recommended viscosity: 15W-40 Above 10 C; 11 Req.
Mobil Delvac
1300 Super 15W-40 Conventional Heavy Duty Diesel Engine Oil, 5 Gallon
Advance Auto Parts - 1089
2121 E Colfax Ave
Denver, CO - 80206
(303) 320-0844
Good luck on the search.