As a precautionary measure and in an effort to better protect Mercedes-Benz engines from the potential long term effects of the increased use of ethanol., DCAG has decided to differentiate the engine oils that are used for Gasoline and Diesel engines. As such, Mercedes-Benz 229.5 0W40 engine oil must be used for all gasoline engines including AMG. Mercedes-Benz recommends the use of Mobil 1 0W40 for all gasoline engines including AMG, with the exception of the SLR. 0W40 engine oil is more effective in resisting the possible corrosive effects of acidic moisture that can be caused by the increased use of ethanol [ biofuel]. The SLR requires specification 229.3, Mobil 1 5W50 oil. To maintain the integrity of the Diesel Particulate Filter (DPF), specification 229.51 must be used for Diesel engines.
Ethanol combustion does not produce more acidic by-products but more water content in the oil . Water in oil enables acidic combustion products to dissolve better in it and these acids may accelerate metal corrosion.
The technical reason why they will have made this distinction is that 229.5 and 229.51 have different TBN values . Total Base Number is a measure of an oils ability to cope with acids. In MB's specs 229.51 must have a minimum TBN of 6 while 229.5 has a minimum of 10. I agree that this is only an issue on very long oil change intervals and to put it in perspective the previous petrol spec 229.3 only had a TBN of 7.
Thankfully our petrol still only has a max. of 5% ethanol although they keep threatening to increase it to 10%