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Looks like the STP-4 does contain a few hundred parts of ZDDP. That would increase ash and maybe some other problems associated with phosphorus and sulfur. With the new requirements coming out soon, STP will be forced to reformulate as will amny engine oil producers.
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Michael D. Holloway
https://simple.m.wikipedia.org/wiki/Michael_D._Holloway
https://5thorderindustry.com/
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