Fuel search?
 

Anyone know if there is a way to set the online destination fuel search default to diesel fuel?

Not exactly online in the sense that you can access it via a web browser. However I have been using this app on my iPhone for years now with good result. You can select what grade or type of fuel it displays:

Gas guru

I use Gas Buddy, works great

^ +1. I use Gasbuddy.com and select diesel fuel. May not show *every* diesel fuel station, but close enough for me.