I see nothing wrong with burning used oil in with the fuel, the large trucking companies have been doing it forever. I burn around 20-25% in my service truck on a regular basis. If you try more than this, you get major amounts of blue smoke and a decrease of power.

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1971 R600 Mack toterhome (SuperDawg)
99 NU-WA Snowbird 34+2 SE 102 5er
If it's not diesel, it's not in my stable!
53 F100 617.951/auto
95 Dodge 3500 Cummins/5 speed service truck