Is this true? 

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Sortof.  Conventional oil causes seals to expand more than synthetic oil does.  So if you switch to synthetic oil, your seals won't be expanded as much as previously with conventional oil, so it will leak.

Fuck, say the car has 110,000 miles, if I used highmileage synthetic would that prevent the leaking or is it just a marketing technique? 

The point of synthetic isn't to prevent leaking. As far as I know, it just burns cleaner and doesn't leave as much crap in your engine, thus preserving your engine better.

I don't know.  Give it a try and see.

or... get your seals replaced?

Hold the phone Mike.  Some rationale for synthetics:

- Maintains its viscosity through a very wide temperature range.  Conventional oils get syrupy at very low temps.  Good synthetics pour like water even below zero.  This minimizes engine wear at cold start since the engine is getting lubricant flow right away.

- Resists thermal breakdown at high temps like when the engine is being run hard for long periods of time.

- Gives excellent lubrication to the engine thereby reducing wear.

Synthetics are great for hard use, low temp operation and high speed/precision internal components ie. turbos.

In the early days of synthetics differences in seal swell rates between mineral oils and synthetics were a big issue.  The oil companies have been tinkering with the additives in the synthetics and this problem has been minimized if you use a good synthetic.  

You should be able to switch from synthetic to conventional and vice versa.  Just know what you are putting in.  Some "synthetics" are actually a mix of conventional and synthetic lubricants.  The big players have different formulations for full synthetics ie high mileage formula etc.  

Look at the websites for big players like Mobil 1.  Depending on your vehicle and your operating environment it may not be worth switching to synthetic.  But I've been using the stuff since the 70s in everything from lawnmowers to airplanes-- love it.  

I was thinking Amsoil for a Wrangler. Think it would have any benefits?

I always run synthetics because IMHO every operating environment is harsh.  If you go off road with the Wrangler it's a no-brainer.  If you live in a very cold or very hot climate it's a no brainer.  If you sit in city traffic or make frequent short trips it's a no brainer.  That about covers everyone in the whole country.

I know Amsoil has been around from the beginning of the synthetic revolution.  That said I am not really familiar with their oils.  I have been using Mobil1 for decades (not because it's better than any other, just force of habit I suppose).  Take a look at Amsoil's website and poke around to see what others have experienced as to switching from conventional to the synthetic.

If you switch monitor oil level religiously especially in the beginning.

My old Chevy pickup may she rest in peace (tree fell on her) was hitting factory new engine compression specs at 180K miles.  The '79 Mercedes had 610K miles on the original gas engine when she was totaled in a wreck.  Just two examples of engines running full synthetic.

Chuck apparently knows a hell of a lot more than I do about oil. So ignore everything I've said and listen to him.

I've heard that before and I've talked to some knowledgeable people and I have to agree with Chuck.

I use Royal Purple synthetic oil and will never go back. My pickup runs smoother and idles lower. I've also noticed an improvement in fuel mileage after I changed out the engine and rear diff fluid with RP. I've gained 1 MPG city and 3 MPG hwy. My dad got 1 MPG hwy. So it's not any crazy gains and may not be typical, but every little bit helps right?

With a high quality synthetic oil and a filter you can easily go much longer on oil changes. I go about 12k miles between changes with RP.

I also run synthetics in the differentials and transfer cases, never had a problem.  Never noticed mileage increases but I wasn't keeping track.

The increased oil change intervals offsets the additional expense of the syn. oil too.

If you run a winch on the Wrangler the syn. is great for lubing the cable.  Stays on, prevents corrosion and is way cheaper than my good gun lubes.

The answer is "That depends on if your seals have shrunk, been erroded away by mechanical wear or chemical attack".

I'm going to assume you're familiar with engine sludge for the rest of this post, if you're not this article explains it farily well:

http://www.nytimes.com/2007/02/04/automobiles/04SLUDGE.html

Your engine has various gaskets in it. You will hear some of these refered to as seals. Over time, this sludge may build up on the seals and prevent oil from reaching them. Because the seal is kept dry, it shrinks. When this condition occurs, the only thing keeping oil from leaking through the seals is that sludge that built up over the seal. Oil can become abrasive at a certain point. Think of it like getting water and sand in your boots, then working another 4 hours. It will physically errode away at your feet. Chemical attack occurs when the oil becomes slightly acidic or compounds are introduced into the oil that act as a solvent on the seals. Think of when you throw gasoline or another hydrocarbon on styrofoam.

If you were using conventional motor oil and switch to "synthetic", an issue can arise due to synthetics typically having a greater detergent property. In layman's speak, it means it cleans and prevents sludge better. If your seals have shrunk, and it cleans the sludge away that was built up over them, then oil will leak through the gap.

In terms of expanding seals, most oils including synthetic do not expand seals any more than 10% if that. I'm gearing up for my own mini study with better empirical data than the brief glance this individual made:

 

http://www.bobistheoilguy.com/

If you need or want me to break it down to the chemical level I'll be more happy to.

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