Some on this board claim with the fuel available today that 26 degrees is a better setting. So if the timeing is 2 degrees plus, keep your money and count your blessings. If it is 2 degrees minus see how it runs before making a dicision. Sounds like the mechanic is an honest guy that is trying to give you the best value for your money spent with him. I would tend to trust his judgement unless the drivability is a problem.

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Sedecrem

82 300SD 290k(needs engine swap)
79 240D 354k with new heart transplant w/220k
82 240D w/auto tranny soon to be manual 4 speed