Originally Posted By: wemay
If an oil is certified SN and even GF-5, it must meet usage requirements of all oils meeting the same specs, and currently, most HM oils do. Every additive within a spec must remain inside an approved range. Although the elastomers (conditioners) are increased for HM oils, they too must fall within this range to be certified API SN. You can use these oils from new.
That's true. All the SN, dexos1, 229.5, A40, etc, whatever specs an oil meets, requires the oil to be tested on a number of different kinds of seal materials to make sure they don't swell or shrink them too much. This is a fact that contradicts all the hunches & guesses from some people on this thread making bad assumptions. Bottom line: If any High Mileage oil you're looking at meets the spec your engine needs, then it will treat the seal materials properly from a swell & shrink standpoint.