As some of you know I'm a recovering fuels and lube engineer. I would NEVER add used motor oil (or virgin motor oil, for that matter) into the fuel system. Running the contaminants held in suspension back through the engine just isn't wise.
I do add 2-cycle oil - which is designed to be combusted - into the fuel to compensate for the loss of sulfur's lubricity.
'94 Barth 28' Breakaway M/H ("StaRV II") diesel pusher: Spartan chassis, aluminum birdcage construction. Peninsular/AMG 6.5L TD (230HP), 18:1, Phazer, non-wastgated turbo, hi-pop injectors, 4L80E (Sun Coast TC & rebuild, M-H Pan), Dana 80 (M-H Cover), Fluidampr, EGT, trans temp, boost gage. Honda EV-4010 gaso genset, furnace, roof air, stove, microwave/convection, 2-dr. 3-way reefer. KVH R5SL Satellite. Cruises 2, sleeps 4, carries 6, and parties 8 (parties 12 - tested).
Stand-ins are an '02 Cadillac Escalade AWD 6.0L and an '06 Toyota Sienna Limited.