Personally, I'd find another garage / mechanic who'd actually take the time to inspect and report on what faults the car has.
Oil leaks can be a pain to find but it's not that difficult. Usual suspects are rocker cover gasket, tandem pump seal, leaking charge air hose seals ( if it's a diesel ). 10 minutes with a pressure washer and degreasing agent to clean it off and maybe half an hour of running the engine usually gives you an idea of the problem area.
You don't say which engine it is, is it petrol or diesel.
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