hamish:

Most of these older W124 Chasis 86-90 require
whats called a valve job, MB mechanics refer to this a a big "tune-up" for these cars, but thats depending on the oil consumption on your car.

Check the plugs, if they seem to be getting wet by oil, or their dark, you might be due for a valve job--big bucks!!($1400) if the oil consumption is not that bad, try to change the plugs twice a year...

Agron
1989 300E