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A truck mechanic told me that drivers have been rumored to urinate in the DEF tank to bring the level of urea up to the point that it can be driven.

Sounds like an urban legend. DEF is 325 g/L urea. Urine is roughly 9.3 g/L urea. Not a great substitute.

I think they're taking the piss.

Well I suppose that's unintended comedy in the naming then.