Thank you JAG for the additional info
…...Well, the SM spec stipulates Zinc and Phosphorus content of 0.06% to 0.08% (600 to 800 ppm). GM EOS contains 5762ppm Zn, and 6221ppm P, or 0.6%. That means in a quart of EOS, you've got about 0.192 ounce of Zn and P. It's been well documented that a 0.10% to 0.12% concentration of Zn and P is optimal for flat-tappet engines, for example in SAE papers 770087, 831760, and 2004-01-2986.
To achieve 0.11% Zn and P in a 5-quart system, we would want 0.176 ounces of Zn and P. There's 0.192 ounce in a quart of EOS, but there's also 0.08% Zn and P in a quart of SM engine oil (actually, most of them run a little on the high side, as it seems, but we'll use the 800 figure). In 5 quarts of SM engine oil, therefore, you've got 0.128 ounce of Zn and P. Therefore, you're a little under 0.05 ounce short of what you want. Doing out the math and remembering that I've been rounding pessimistically, 8 ounces (1 cup, i.e., half a pint) of EOS would bring most any SM engine oil back within the optimal Zn and P content, which means that a quart of EOS would last you four oil changes. .....
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