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Synthetic Oils and Leaking
There appears to be something to this issue. Some logical explanations have been offered up for the phenomenon.
The following is sort of a sales blurb for a Schaeffers Lubricants product, but like many sales blurbs found on oil sites, contains some interesting insights. Scroll to the bottom and look for, "One of the important things that is happening here is that when you clean a seal..."
In other words, worn or failing seals may have their sealing abilities embellished by sludge or other crud building up around them and acting like a stop leak, and an oil that offers superior cleansing capabilities may well clean that area around the seals and a leak may result. The claim on the above site is that their seal conditioning product may increase the lifetime of function for such seals. It brings up other points as well.
One is that the same contaminant which has been acting as a stop leak has also been depriving the seal of needed nourishment, hastening its demise. If true, this could explain why people switching to an oil that cleans better at say 20,000+ miles, get leaks that someone who has been using a synthetic or other oil with good cleansing capabilities does not.
Another explanation proposes the theory that HD 360 oil contains seal expanders, and when those are leeched out of the seals, their functionality is reduced and a leak results. Does HD oil contain seal expanders? HD isn't saying, so it's just an unproven theory.
The claim that some synthetics are just "thinner" oil. But since the oils are API certified, they are tested for viscosity and shouldn't actually be thinner at operating temperature.