Tech Session on Zinc in Oil
There has been a lot of confusion in the last few years about the lowering of zinc and phosphorus levels in modern oils and how these lower levels relate to classic and performance engines using standard flat tappet lifters – that is, just about every car built before the Eighties. The concern involves the use of the new lower zinc/phosphorus-content ILSAC (multi-viscosity) oils, readily available on shelves at auto parts stores everywhere, and how compatible they are with these older engines.
When anyone mentions zinc, they are actually referring to zinc dialkyldithiophosphate, a compound invented by Castrol for use in mineral-based oils or zinc di-thiophosphate (ZDTP), which is normally used in synthetic oils. Both have been used as an anti-wear ingredient in engine oil for many years. The zinc and phosphorus ingredients appear to be most effective when they are used together. ZDDP/ZDTP is one of many additives that are put into conventional motor oil to improve its lubrication qualities. Other ingredients such as boron and molybdenum are also added as lubricant enhancers.
What was discovered through oil testing by several engine component manufacturers is that many older engines experience a short period of time during engine start-up where critical lubrication is insufficient between metal-to-metal lubrication points when using modern oils with reduced amounts of ZDDP/ZDTP. These same enhancers unfortunately have their downside: The phosphorus in this compound creates carbon buildup in engine bores and valvetrains, and both compounds can also lead to the early demise of catalytic converters. For this reason, the industry has been phasing out zinc and phosphorus levels since 1994, when the American Petroleum Institute’s SH designation became the industry standard, and levels have been further reduced in each subsequent API rating for engine oils. Manufacturers have tried adding more boron to offset the effects of the reduced zinc and phosphorus levels; however, the dry start protection does not measure up to those using more ZDDP/ZDTP. This has opened up a whole new market for zinc/phosphorus additives for oil and many camshaft and engine manufacturers now recommend that an additive be used in initial break-in and for regular use.
All engine oils are rated for viscosity by the SAE as well as additive content by the API; passenger car ratings are two-letter designations that start with “S.” Heavy-duty or off-road equipment ratings start with “C.” The current API oil rating for passenger cars (gasoline engines) is SM and for trucks (diesel engines) CJ-4. Within these designations, you can determine how much zinc and how many other chemicals are present in the ILSAC (multi-viscosity) oils. These levels do not apply to straight-weight oils. If levels in the ILSAC oils are too high for the API specification, they cannot be rated for the current specification unless the container specifies “for racing or off-road use only” or “for use in classic cars.” This has caused oil companies to reduce levels of many additives, including zinc and phosphorus, to the required maximum in order to meet the current specification. Listed here are the current specifications for maximum amounts of additives to achieve the API ratings. P is phosphorus, Zn is zinc, and B is boron. Each figure is total parts per million of additives. These can also be roughly expressed in percentages by multiplying by .0001 (1301 PPM = .13 percent, 994 PPM = .099 percent)
Most engine and engine component manufacturers recommend zinc and phosphorus content of more than 1,200 PPM for break-in; in fact, many will void warranties on camshafts or crate engines if this minimum is not found in the oil sample you supply when returning broken parts for warranty. For this reason, many manufacturers produce their own zinc additives or oils with supplementary zinc included; GM even offers its own break-in oil with additional ZDDP. With respect to readily available oil, you can see from the chart that, if you can find oil still on the shelf rated SJ or SL, you can use them, but you are right on the cusp of voiding a warranty. New SM oils are just not going to cut it unless they have a zinc additive to boost the rating and one of the zinc supplements should be used with these oils or oils containing additional ZDDP additives are recommended. Some enthusiasts have recommended using commercially rated CI-4 15W40 diesel oil to meet the zinc and phosphorus additive requirement; however, CI-4 is an old specification and hard to locate. You can see that the CJ-4 specification that now supersedes it is well below acceptable levels. Our best recommendation is that you contact your oil supplier for exact additive contents. Many straight-weight oils do not have to meet the ILSAC API specifications to be sold as SM or CJ-4, so this may be an alternative. Classic car oils with elevated levels of ZDDP/ZDTP are also being offered by many suppliers. Regardless, if you are purchasing off-the-shelf oil for your classic car, ILSAC multi-viscosity oils rated SM or CJ-4 should have stated zinc and phosphorus additive supplements for use in older engines or an additional separate additive should be purchased and used with the new oil. As the new API rating SN becomes available in the next year, even more caution should be taken as the levels will be reduced even further.
Article by Jim O ‘Clare – Brought to you by the NAACC