Found it. This article was written to address specific concerns about modern PCMO performance in older engines with flat cam followers. eni is the modern name for AGIP oil, still has the fire breathing six legged dog on the oil labels. I work as the Technical Advisor - Fluids and Lubricants for the eni distributor here in New Zealand.
Zinc and the modern Passenger Car Motor Oil.
A Passenger Car Motor Oil contains a number of additives to provide its in-service performance. Detergent keeps the engine clean, dispersant keeps contaminants in suspension in the oil until they are drained at oil change, oxidation inhibitor stops the oil from thickening and forming resins and varnishes at high oil temperatures, viscosity index improvers produce a multigrade engine oil which gives good low temperature starting viscosity with excellent high temperature viscosity performance, and anti-foam stops the oil frothing.
But probably the one additive in an engine oil formulation that most people recognise would be the anti-wear additive, frequently represented as the zinc content of the oil. Many engine oil Product Data Sheets will list the zinc content of the oil as an indication of the oil’s anti-wear performance.
It’s actually not the zinc itself in the anti-wear additive that provides the anti-wear effect, it is the phosphorus and sulphur in the zinc based anti-wear additive. But, because the zinc content of a PCMO is easy to measure, zinc content has become the generally accepted indicator of the engine oil’s anti-wear performance. Zinc is present in an engine oil formulation in different forms of a metallo-organic compound, zinc dialkyl dithiophoshate (ZDDP). However, these days the zinc content of a PCMO is not an absolute measurement of that engine oil’s anti-wear performance.
One of the descriptions used to describe a PCMO’s performance are the American Petroleum Institute (API) Service Classifications. The API is an independent industry body. For a PCMO, these classifications start with the letter “S”, indicating service station or spark ignition applications. The different API Service Classifications describe a PCMO’s engine cleanliness, deposit control, oxidation resistance and anti-wear performance. From API SC in 1964 to API SG in 1989, each move from one “S” classification up to the next was a considerable increase in the level of engine cleanliness, deposit control, oxidation resistance and anti-wear performance.
When API SH was introduced in 1994 there was no change in the engine tests used to specify API SH from those used for API SG. However, more stringent testing and manufacturing conditions were put in place. It is estimated that the more rigorous API SH testing and manufacturing regime brought about an overall 10% increase in in-service performance from API SG to API SH. All these API Service Classifications up to API SH specifically addressed engine and lubricant performance, API SH PCMOs generally included zinc levels of up to 1200 or 1300 parts per million (ppm). But API SG and SH were the last PCMO service classifications where the zinc level in the engine oil was any reliable indication of the PCMO’s anti-wear performance.
With the introduction of API SJ in 1996 (there are no API SI or SK classifications) the API Service Classifications became focussed on the control of engine oil components whicht could contribute to exhaust catalyst poisoning. And wouldn’t you know it, one of the main contributors to catalyst poisoning was the phosphorus and sulphur from the ZDDP anti-wear agent! So from API SJ on, there has been a maximum level set for the phosphorus content in a PCMO. Effectively, from API SJ onward, the level of ZDDP in engine oils has had to be reduced and other, non-zinc anti-wear compounds used. The zinc level was no longer an absolute indication of the engine oil’s anti-wear performance.
API Classifications SL, SM and SN all contain specifications for maximum levels of phosphorus and sulphur, and therefore ZDDP, for exhaust catalyst protection. You will have heard of “Low SAPS” (Low Sulphated Ash, Phosphorus and Sulphur) engine oils, these are engine oils which have reduced levels of ash producing compounds and phosphorus and sulphur so that the engine oil gave maximum exhaust catalyst protection. However, higher viscosity grades of these modern API classification PCMOs (SAE 40 and above) are exempt from these limits on phosphorus and sulphur, and therefore zinc.
The level of modern engine oil’s anti-wear performance also doesn’t need to be as high because engine designs have evolved. So while API SJ, API SL, API SM and API SN PCMOs are perfect for modern petrol engines, they may not be ideal for older engines, especially those fitted with flat tappet cam followers.
Because of the high sliding load of flat tappet cam followers on the camshaft in older engines, a higher level of anti-wear agent is desirable to prevent cam lobe wear. The perfect anti-wear agent for this job is our old friend ZDDP. And ZDDP at the level found in older, API SH and earlier, engine oils. Larger engines fitted with flat tappet cam followers benefit from higher viscosity engine oil, such as an SAE 20W-50.
So what does a lubricant distributor do? Transdiesel is introducing API SL classification eni i-Base Formula Super SAE 20W-50 with a zinc level of 1600 ppm. eni i-Base Formula Super SAE 15W-40 also has this 1600 ppm zinc level.
The SAE 20W-50 viscosity grade will give older engines, such as big block American V8s and many other older automotive engines, the extra level of anti-wear protection that their design and operation demands. eni i-Base Formula Super SAE 20W-50 is also ideal for use in the original Mini where the engine and transmission share the same lubricant and SAE 20W-50 engine oil was specified to meet the viscosity requirements of the transmission.