Im in the opposite situation with my 82 currently (locks uncleaned). Just got a new dealer key, my original, which shows some wear goes in/out far smoother, while the new one catches/binds.

I have to wonder if the dirt inside makes up voids/gaps that lets the old key slide in, while the sharp edges on the new one then get caught because they want to move the tumblers to their correct position which the dirt prevents (or causes over-movement).

Remove the dirt, the old worn key cant cause the tumblers to move far enough... they are ever so slightly misaligned and thus cause the scraping. The cleaned tumbler with new key works as is normal.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)

I think this is true. The new key did not work as smoothly as the old until I cleaned it. This difference is important for those people whose keys are not turning and have decided to flush out their lock with WD-40 in a last ditch effort to get one more turn out of it to remove the cylinder. A new key might be helpful to do that once it is flushed out.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13