I guess the pit would be like a large drum burried in the ground, and so any oil spilt (but not rainwater) will run into the drum. Either that or get some silicone rubber and seal up the bottom 2 foot of your shed.
Or you could not spill any oil? (anyone not expirenced with oil will think its an ok idea. Anyone expirenced will question if I have any expirence with oil, or will be laughing at such an unfeasable idea)

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cheers
Chris.
1990 landcruiser 80, 1HD-T two tank, copper pipe HE+ 20 plate FPHE, toyota solenoids and filters. 1978 300D, elsbett one tank system.