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xyrodon how big is the oil leak? You said it was a serious leak but if its just "sweating" as your mechanic is, then that's nothing. My car has had the small seepage behind the camshaft seal its whole life and I'd say it does nothing to affect the longetivity of the car, as long as you're keeping good oil in it.
The things that I'd say would affect the longevity of the car more is wear on the drivetrain components, body, mounts. Most cars are due for having mounts and bushings replaced or upgraded by 100,000 miles. Plus years of driving through salt and slush, you're likely to have parts rust off your engine rather than run into trouble with a miniscule leak.