Diesel is just a thin oil with detergents added, I'd probably use petrol in the box if I wanted to get rid of all traces of old oil if I was changing from/to something like Redline which doesn't mix well, just fill it up and lift off the ground and spin the box a few times to dilute what is in there but otherwise I wouldn't bother.

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Beaten up old 98 Gen 2 Liberty, 13.0, stock V3 STI running gear.
07 Transit motor home
69 VF & 70 VG Valiants needing full restoration
Yamaha Zooma 50cc rocket hopefully going with the Transit.