I agree with the Daves above

The oil pump is the least wearing part of an engine as it the best lubricated part of an engine

It's extremely rare for an oil pump to be worn in a good engine so the only time to replace the oil pump is when rebuilding an engine. In fact when rebuilding an engine you should always replace the oil pump even if the old one seems fine. But the oil pump drive problem mentioned above is a common failure in these engines. It happened to me twice, first in my 124 and killed the engine and then again in my Silver Sport but I knew better by then so the engine was fine. I now always replace the drive gear when I get a car as at under ?20 for the gear and about 10 minutes to change it's very good insurance
