Took it to halfords for a Mercedes compliant service. All filters and most oils including coolant replaced for £264. Happy man as this was a lot less than MB and a number of indies
For the OM651 cdi, its 6.5 litres.And there are plenty of posts here on how to go about it if you are doing it yourself. But a word of advice, when refilling it , just put 6 ltrs in, drive it and let it sit overnight, Check it again next morning ( before starting the engine) and you will see the true level. Some people fill the full 6.5 ltrs after draining it, without checking how much has actually drained,,,,,and if it does not drain the full 6.5 ltrs( it wont...always a small amount remaining ) then filling the 6.5 ltrs will actually overfill the sump....and these engines are very sensitive when it comes to the proper oil level.