From Penrite site
An engine oil that contains about 0.1% phosphorus or higher, will easily provide the required anti wear properties for older engines.
The step from API SH to API SL was accomplished by a combination of new additives or adding additional anti wear and anti oxidant to existing blends. These were not phosphorus based, but used organic molybdenum additives, to keep phosphorus levels at 0.1%.
Now we have API SM – for the first time, the limit on phosphorus is from 0.06-0.08%. There are industry concerns about the applicability of these oils in older engines. However, the limit only applies to 0W-20, 0W-30, 5W-20, 5W-30 and 10W-30 oils (so called “ILSAC” grades). Any other grades are exempt from this.
Therefore even blanket statements about API SM oils will be incorrect and further research will be needed by the end user.
There is one other factor with non-ILSAC oil grades. If they also have the European ACEA A2/A3 with B2/B3 or B4 performance levels, phosphorus levels will also be at 0.10 % to 0.12% as their tests have been more severe than the API for some time. Hence an oil that is SL (SM)/CF/A3/B3 also well exceeds the anti-wear requirements for older engines.
The irony is that API SF and SG oils formulated in recent years usually have phosphorus contents of around 0.08% (usually 0.1% maximum) anyway due to other advances in technology, unless the blender chooses to add extra additive.