Due to the increased sales of diesel passenger cars and SUVs, including those recently caught gaming the emissions system, a greater number of traditional, on-highway filling stations now have diesel’s green pumps. However, pumps that offer diesel exhaust fluid (DEF) continue to be few and far between.

Since 2010, many diesels are required to have DEF, a mix of two-thirds water and one-third urea, to run their higher-emissions controls. As a result, highway truck stops stock DEF at diesel pumps, while automotive parts shops and dealerships offer DEF jugs at a higher price. As with fuel, jobsites also may have bulk DEF for on- and off-road vehicles.

Finding DEF in the wild can be a hassle, especially as diesels are designed to slip into a “limp” mode when the DEF tank runs dry. For that reason, some diesel-truck users keep a jug around, just in case.

We felt the pinch on Interstate 70, driving a one-ton Ram 3500 pickup (see story, above) to Indianapolis: The diesel-fuel and DEF gauges were low. We took the Ram past several highway work zones before filling up in Warrenton, Mo. We bought about 20 gallons at a cost of $2.09 per gallon, or roughly $41.

We were interested to see whether DEF was offered at the same local highway pump. There was none, so we had to keep hunting. Enter Flying J, a large truck stop a few miles away. It had DEF in the pump but only on the heavy-duty trucking side.

We filled up the DEF tank with about 4.5 gallons at $2.79 per gallon, or $12.56. We later filled up again with AutoZone’s 1-gallon DEF jug, which cost $8.01.