A lot of "seal friendly" comes from additives. True synthetic oils don't swell rubber as much as traditional dino oils do, and less swell can spell a leak to old seals. The solution is in the additives, synthetic oils have additives to swell the seals as much as dino oil, but it can vary.

There is no danger in a fluid with less swell, it will leak and require the old type oil again to properly seal. If the oil has MORE swell, it can cause the seal to wear and you can't go back.

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Gone to the dark side

- Jeff