This can sometimes become a controversial subject so I'll say upfront this is simply my experiences and I don't dispute some of the horrible things others have experienced........
I've burned a lot of used motor oil, vegetable oil, kerosene, home heating oil, transmission fluid, hydraulic oil, and yes even old gasoline. IMO, the success or failure you experience burning these alternate fuels depends on several things.
-Percentage of alternate fuel & regular diesel is a big factor. I never exceed 50% alt fuel in the mix and prefer closer to 67% diesel / 33% alt fuel.
-All filtering and mixing must be done before it's dumped into your fuel tank. Some folks simply dump unfiltered & unmixed used motor oil in the tank which is bad news. If you have the luxury of time, natural settling is your best friend. Simply let gravity do the job prior to filtering & mixing.
-Only use in vehicles with two fuel tanks. Mixed fuel is kept in one tank, straight diesel in the other.
-Always start & stop the engine with regular diesel and never introduce mixed fuel to the engine before it reaches normal operating temperature. I also never use mixed fuel on short runs. Most of the mixed fuel I've burned occurred on 100 + mile trips. Again, starting and shutting down with regular diesel in the fuel system.
-Last & probably one of the big ones, engine condition plays a huge role regarding your success or failure burning mixed fuels. Engines in poor condition will burn less efficient and become problematic quick.