A PASSENGER CAR MOTOR OIL (PCMO) CONTAINS A NUMBER OF ADDITIVES TO PROVIDE ITS 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 are aware of seems to be zinc. Many engine oil Data Sheets will list the zinc content of the oil to give an indication of the product’s anti-wear performance. It’s actually the phosphorus in the zinc based anti-wear additive that provides the anti-wear effect. But, because the zinc content of a PCMO is easy to measure, zinc content has become the indicator of anti-wear performance.
These days the zinc content is not an absolute measurement of engine oil’s anti-wear performance. Zinc was widely used in an engine oil formulation because different forms of zinc dialkyl dithiophoshate (ZDDP) were used as the main anti-wear agent in the engine oil formulation. Different forms of ZDDP have higher and lower anti-wear performance and lower and higher anti-oxidant performance. It was a bit of a balancing act, what you gained in one characteristic you lost in the other. Some zinc based anti-wear additive packs acted at high surface temperatures while others were activated at lower temperatures.
PCMOs have their performance described by 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. API SA described engine oil which was basically straight mineral oil and nothing else. API SB required a small level of anti-scuff performance and oxidation resistance. API SC was introduced in 1964, SD in 1968, SE in 1972, SF in 1980 and SG in 1989. From API SC to SG, each move from one “S” classification up to the next was an 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 oil performance tests from those of API SG. However, stringent manufacturing and testing conditions were put in place. It is estimated that the more rigorous API SH testing regime brought about an overall 10% increase in PCMO in-service performance from API SG to API SH.
But API SG and SH were the last PCMO service classifications where the zinc level in the engine oil was any reliable indication of anti-wear performance. These PCMOs had zinc levels of up to 1200 or 1300 parts per million (ppm). From the introduction of API SJ in 1996 the API Classifications were more focussed on the control of engine oil components that contributed to catalyst poisoning. And wouldn’t you know it, one of the main contributors to catalyst poisoning was phosphorus from ZDDP! So from API SJ on, there has been a move away from zinc based anti-wear agents, including limiting the phosphorus content of a PCMO to below 0.1%. The zinc level of an engine oil was no longer an absolute indication of that engine oil’s anti-wear performance.
API Classifications SL, SM and SN all contain reduced levels of phosphorus and sulphur, and therefore zinc, for catalyst protection. You will have heard of “Low SAPS” (Low Sulphated Ash, Phosphorus and Sulphur) engine oils. There are even zinc-free PCMOs available now which pass the anti-wear and chemical requirements of today’s oil classifications.
So, from the mid-1990s, zinc level could no longer be taken as a reliable indication of the anti-wear protection the engine oil provided. The level of engine oil anti-wear performance also doesn’t need to be as high in modern engine oils 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 concentration of previous API SH engine oils. Larger engines also seem to benefit from higher viscosity engine oil, such as an SAE 20W-50.
For more information on the right oils to use in your classic car, get in touch with TDX’s lubricant technical team: lubesupport@TDX.co.nz