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I BELIEVE much of the mineral v synthetic debate re leaking seals stems from how mineral oil reacts with seals and causes them to swell slightly over time. Switching to synthetic (or different viscosities) can cause the swelling to abate and create leaks where they weren't previously. OTOH, if synthetic is used from the getgo (on a new engine or after a rebuild), seals will not swell/have this issue. Zinc and its beneficial effect for flat tappets/etc. is another story altogether. I used to run mineral until my top end overhaul and have been running synth with mineral leakage for the past 80k since my top end
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