Me again...
The Api thing has been bugging me.
If a superior oil meets a lower (considered outdated) api spec than a lesser oil, there will be a reason, which will probably be one or more ingredient(s) not permissible or not permissible at the same levels in the newer spec, for 'modern' reasons, that the manufacturer has included for performance reasons which deny it the later classification.
My research has come up shy, but what I have found is that SN only permits 20% less phosphorous than SL. If youve heard people banging on about this 'magic' ingredient zddp in oil, that is Zinc D(something)D(something)Phosphate, then the phosphorus levels are to do with the proportion of this substance. Thus I would make an educated conclusion that this is why Edge (& others) that are simply A5/B5 with few other specs only, have too much zddp to meet SN and other manufacturers specs, thus are only rated SL. In the right engine, with appropriate catalytic converter, this will provide improved wear protection over the later specs. As such, like mentioned earlier, using an older spec oil can (where appropriate) be better than a later one.
I suspect this is (at least one reason) behind Volvos call for simply A5/B5 in an era of 'cleaner' oil specs.
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