Despite the fact that I had advised against burning used Motor Oil as Fuel I tried out of curiosity; 2 quarts of used Motor Oil in my Volvo Diesel.
(However, I filtered it through a 2 nominal micron Filter Bag before it went into the Tank.)

It makes the Exhaust gas stink! And, it stinks even after the Tank was refilled with only Diesel Fuel.

Motor oil has additives in it to retard burning and used Oil can have acid in it.

It also has an Ash content to it. Ash content is actually Metal.

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84 300D, 82 Volvo 244Gl Diesel