Before I answer this question it should be known that it is recommended to discard leather that has been saturated by diesel fuel because of the fire hazard it presents if it is ever exposed to open flame or sparks.
If you have a .pdf reader such as Adobe Acrobat Reader you may view the Material Safety Data Sheet at the following url:
Generally for any oil based stain on leather it is best to cover the affected area(s) with talcum powder. Once the areas are covered well, you must let it sit for about 24 hours. Once the powder has had time to sit you may brush it off briskly. This method may require some patience as it may take a few attempts to fully remove the stain.
I hope this has been helpful for you.