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I haven't experienced that. After replacing the RMS and other leaks on my 4.0L, I've continued using Mobil synthetic high mileage and its been leak free.
For some of us, synthetic oil DOES make the RMS leak.

If synthetic oil is working for you with no leaking, that's great.

I guess it's just a matter of whether or not you have an oil leak.
 

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Synthetic oils run cleaner and will clean out the crankcase after a few changes on a decently maintained engine (over-sludged is a different story). Better detergent packs. Cleaner engines means less dirt to slow leaks to a weep instead of a drip. Synth flows better and therefore will mechanically clean better too.

Cheaper sythns also tend not to swell gaskets and seals like conventional oils. Modern replacement gaskets are truly compatible with both conventional and synth oils. This has been true for many many years.

Engines that have been pushed hard, overheated or abused repeatedly over the years will also result in work hardening of both the gasket and gasket surface.

Every gasket design has number of thermal and pressure cycles and/or number of hours as it's basis for a design. We'll never know what that is since it is considered trade secret info. Most 4.0 engines are well beyond design life (time or mileage or hours)...so now it is a waiting game, but not a bad one as the 4.0 were well over designed

Gaskets do have a shelf life and will dry out especially if stored in hot, dry temps... so be careful about the age of the gaskets (just like why vehicles that don't run tend to have bad belts, leaks, and tires).

Synth does not cause leaks - it highlights weak points in sealing/gaskets and surfaces due to age, dry out, scoring on gasket surface, etc. A bad/old gasket or poorly sealed engine(design) is still a bad gasket or poorly sealed engine...it's not the oil that is the issue.

High mileage (synth or conv) oils contain ad packs that can slow leaks and extend gasket/seal life by swelling gaskets that are beyond/have reached end of life.

If a conventional oil results in less leaks - great. Run it. Just know that cold temps, overall engine life, fuel economy, etc will be impacted, statistically.

Source:buddy who was a lubrication engineer.

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