I have never run used grease. I did use 3 gallons of fresh soybean oil in an empty tank as an experiment.

It ran well. Very quiet, little smoke, almost unnoticable loss of power, and smooth running. The exhaust smells exactly like burnt popcorn.

If diesel were to ever go over $2.75 per gallon, I would be happy to switch to storebought soybean oil. Until then, I'll just have to settle with standard petro-diesel.