I googled it and here's what I came up with.
Does it stink to change your oil?
With some brands, there can be an unpleasant scent immediately following an oil change. The most noticeable time this condition occurs is with the engine at idle speed and only during the first 500 - 1,500 miles after the oil change. Diesel Fresh helps neutralize this condition.
Diesel Fresh is used in the initial factory oil fill of the Cummins engine. It is formulated with premium quality 15w40 CI-4 diesel oil and a proprietary lemon odorant. Compatible with all CI-4 and CI+4 diesel lubricants.
The four-ounce Diesel Fresh bottle treats up to 12 quarts of diesel engine oil. The product has a lemon scent.
Borrowed from 3rd strike performance's site.