As I understand it the 229.51 is a low SAPS (SAP is Sulphur, Ash and Potassium) oil designed for vehicles with DPFs (diesels). SAPS is the additive package used to reduce wear in engines but as it contains some compounds harmful to DPFs it cannot be used in high concentrations. Accordingly the 229.51 oils have a higher detergent content (used to dissolve deposits found inside the engine) and an anti wear package consisting of a different set of compounds, mainly ZPPD (zinc diallkyldithiophosphate), Calcium Suphonate and sometimes Molybdenum Disulphide (MoS2).
ZPPD in high concentrations can be harmful to catalytic convertors but in 229.51 oils the concentration is at a level that isn't harmful. There are retail packs of ZPPD additives available under various trade names.
SAPS were used predominently as they were cheap (the sulphur compounds were waste products) and highly effective thus allowing the use of lower quality basestock oils. Low ash oils tend to use higher quality basestock oils to enable the relevant protection to be maintained.
So what does it mean?
It means the additive package may be slightly less effective but that the oil basestock may be better quality to offset it. In reality the effect on an engine will be negligible between the two as long as they are approved by the manufacturer.
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