Was it a hairline crack you found with it, @ross.lily? I had a tiny coolant leak on an old bike engine that was impossible to spot until that dye lit it up bright yellow. Tried cleaning and looking for weeks before getting one of those kits. It showed the problem in like ten minutes. Total time saver for sure.

Yeah, that's the classic trap with switching fuels. You need to track your fuel use daily and call the supplier when you hit half a tank, not when you're empty. Set a hard rule for the yard: if the biofuel truck is more than a day late, switch everything back to regular diesel immediately. Keep a backup tank of diesel on site at all times for this exact reason.