Should I switch to synthetic oil or not?

I’ve been driving for almost 40 years now (over 30 vehicles) and with the exception of one Jeep for a short period of time, have never ran synthetic oil in the engine of any of my vehicles and have exactly zero oil related failures.

FWIW, that jeep that saw synthetic oil did immediately start leaking out of the rear main seal but I’d chock that up to coincidence and not the oils fault.
 
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Won't make any difference at all other than the placebo effect.

Your 4.0 has more in common with a tractor than a Ferrari... literally.
 
Why do OEMs factory fill with synthetic oil?
Do they? I think some just want the vehicle they are selling to appear to require minimal maintenance. When we bought a JL the dealer told us to change the oil every 7K or something like that. We changed the oil at 1000 as I still have an old school brain that says a break-in engine should get fresh oil the first 500-1000. Get the shavings out. After that, we will change the oil every 3-4K most likely. Too much? Maybe.
 
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FWIW, that jeep that saw synthetic oil did immediately start leaking out of the rear main seal but I’d chock that up to coincidence and not the oils fault.

I changed to a synthetic after purchasing with 95K on the odo, and I also developed a rear main seal leak. At the next oil change, I used regular old Valvoline 10w-30 conventional oil and there was no more RMS leak. Also coincidence? Probably not...
 
Regardless of oil type the first thing to go is the additive package in the oil. Generally a conventional oil will start to break down sooner in high heat. We are talking HIGH heat that a 4.0 should never see. And syn has a more consistent molecule size than dino. And that makes it hold up better.
As far as folks that like to call it a tractor engine, I think most have no idea what a tractor engine is expected to hold up to. ( Fun fact: Ferrari was a tractor company long before it started making expensive cars. Many farms in Europe still run them. There are also a fair amount of Ferrari tractors used in USA too.)
A modern tractor engine is a twin turbo design that revs higher than the 4.0 and is kept at those higher rpms for hours and hours and hours a day. Every day. Every week. Let's see those fancy "Ferrari engines" try that. And the high torque loads that get transmitted to the crankshaft would blow up most all fancy motors made today. Or the shock loads. I could go on.
Also, I believe ALL new vehicles have had the break-in already done at the engine factory. In the past that may have been only long enough to get it to the dealership, but today they are pre-run on basically a test stand and have had the computer ensure fuel oil coolant rpms and a whole list of things tested to ensure good longevity and performance.
Use syn or dino oil. It is up to you. In the past syn was much more likley to cause seal leak issues and dino more likley to cause sludge issues. But today neither is common. Using a "high milage" oil gets you a higher wear protection (usually zinc in some form. Like break-in oils) and an additive package that helps slightly seal swell to ward off leaks.
 
( Fun fact: Ferrari was a tractor company long before it started making expensive cars. Many farms in Europe still run them. There are also a fair amount of Ferrari tractors used in USA too.)

I can't find any connection to Ferrari cars and Ferrari tractors. Lamborghini was a tractor company before being a car company.. Enzo insulted Mr. Lamborghini so Lamborghini cars were born.
 
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( Fun fact: Ferrari was a tractor company long before it started making expensive cars. Many farms in Europe still run them. There are also a fair amount of Ferrari tractors used in USA too.)

I can't find any connection to Ferrari cars and Ferrari tractors. Lamborghini was a tractor company before being a car company.. Enzo insulted Mr. Lamborghini so Lamborghini cars were born
 
The biggest advantage I can see with running synthetic is it does not brake down as fast as conventional oil. Depending on which manufacturer you use there are certain mileage warranties that they will honor so long as you keep well documented changes.

This is how I run my fleet. I use synthetic because I can run longer between changes. That is the key. My Audi is wildly expensive special oil, but I use it because I could go 10,000 between changes. I go 6K. I never mess with oil and I don't cheap out at all. I have had several cars run over 200K miles even before that was common. You can run both and get the same result, but synthetic you can go longer.

That is how I see it and my mechanic that is a very good friend of mine.
 
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There's some misinformation and/or misconceptions in this thread in my personal opinion.

First, synthetic and conventional motor oils collect dirt, naturally occuring acids, and combustion byproducts at the same rate and the oil filter cannot filter all of of those out. So if one type or the other lasts longer, both types still gets just as contaminated at the same rate so they should both be changed at the normal and suggested intervals.

And while synthetic engine oils are definitely better in extreme cold conditions due to their better flow characteristics in those conditions (like below zero fahrenheit), those benefits largely fall away in temperate conditions. I live in a temperate climate and have run nothing but conventional engine oils for literally over 50 years and have never (!) had an engine failure even though I tend to keep vehicles for close to or over 200k miles.

Conventional lubricants definitely have their place in some assemblies. The ring & pinion gears are proven to run cooler when lubricated with conventional GL-5 gear lubes. Which is why companies like Currie and Revolution gear require conventional gear lubes or their warranties are subject to being voided.

I do run synthetics in parts of my Jeep where they make sense, like in the transmission, and transmission but I'm not a 'synthetic is best everywhere no matter what' advocate. I choose to run conventional lubes in the engine and axles for sure. :)

For just one example, this is what Revolution Gear says at https://www.revolutiongear.com/ring_and_pinion_break_procedure

"Recommended gear oil is an 85-140W, Non-Synthetic, GL5 grade or Higher, Name Brand Oil "
 
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@Jerry Bransford why do axles run cooler?
It's the ring & pinion gears that they discovered run cooler when lubed with a conventional gear lube. Surprisingly to everyone, synthetic was acting more like a thermal blanket and it wasn't sinking (extracting) the heat out of the gears as well as a conventional gear lube did.

What fooled everyone for so long was that the diff covers were cooler with synthetic lubes inside when their temperatures were measured with an IR gun. It turns that that was happening because the synthetic was holding the heat in the gears more and not extracting it. They only figured that out when they figured out how to build a jig so the temperature of the gears could be measured directly.

It's counter-intuitive when you measure the temp of a diff cover holding synthetic and see that it's cooler, which led pretty much everyone to believe the synthetic was causing the gears to run cooler. In fact it was the opposite and it took a lot of testing and re-testing to convince everyone what was really happening.

Companies like Currie were seeing more R&P failures in their custom axles than expected and it was trending up. It turned out that was because more and more were starting to run synthetic gear lubes which they hadn't thought of at the time. Just that more clients were reporting they were using synthetics which alarmed no one at that point. That's when the investigations started and it took a while to figure out what was really happening. John Currie shared that with me at a local 4x4 show and he said he was as surprised as as everyone. They changed their warranty to require a conventional gear lube in their axles to maintain the warranty and their warranty issues went back down to the expected level.

For normal axle use it doesn't really matter if you run synthetic or conventional. But for gear break-ins of aftermarket gear sets that aren't broken in and axles in hard use like Currie's are, it does make a difference.

I had a set of brand-new RGA gears in my rear axle fail two years ago after a very conservative break-in. Not trusting myself and the friend who did my regearing to install the replacement gears again with that kind of problem, I took my Jeep to one of the best gear guys in SoCal. As soon as he got my gears out he said the gears had gotten too hot and said 'you used synthetic gear lube didn't you'. I had. He told me why not to run a synthetic in my axle, due to my harder than typical street uses, and said especially not to run a synthetic gear lube during the gear break-in. I took his advice and that next set of gears did fine.

Street vehicles do fine with synthetics and so do new vehicles since their gears are pre-lapped before installation and thus need no break-in. Aftermarket gears? I'd use nothing but a conventional gear lube in a Jeep that sees much hard use.
 
For the engine oil, synthetic isn't required for "normal" temperatures. For extreme heat or extreme cold it's beneficial. I run synthetic year round from my engine oil to my grease except for my diffs because 1) I get synthetic Amsoil for the same price or cheaper than conventional 2) my miles are inconsistent and I can't predict if I'll need to change the oil before winter comes or during. So even if it does cost more for me it's cheaper to change oil based on miles rather than by season if it's only been 1k miles.

How many are running synthetic, what are the pros vs cons?

As long as your engine is in good shape there's no cons to running synthetic. You don't have to worry about switching or mixing. Synthetic won't make your gaskets leak if they're not already going to.

First, synthetic and conventional motor oils collect dirt, naturally occuring acids, and combustion byproducts at the same rate and the oil filter cannot filter all of of those out. So if one type or the other lasts longer, both types still gets just as contaminated at the same rate so they should both be changed at the normal and suggested intervals.

While they may contain the same quantity of contaminates, won't synthetic have less by percentage? Per liter, synthetic takes up less physical space to provide lubricity which leaves room for more neutralizers and conditioners. If I understand correctly, this is how a lawn mower that requires 32:1 conventional mix can run just fine on 100:1 synthetic (assuming the carb is tuned).
 
Street vehicles do fine with synthetics and so do new vehicles since their gears are pre-lapped before installation and thus need no break-in. Aftermarket gears? I'd use nothing but a conventional gear lube in a Jeep that sees much hard use.
Just to echo this, we break in many dog cut sequential boxes on track cars with conventional and then switch to synthetic once fully broken in. In all honesty I’m not sure the science behind it, just that’s how I was always taught to do it.
 
I can't find any connection to Ferrari cars and Ferrari tractors. Lamborghini was a tractor company before being a car company.. Enzo insulted Mr. Lamborghini so Lamborghini cars were born.
Actually almost All auto companies including Porsche, some French names even autocorrect can't decipher from my spelling, and several English companies made farm tractors. After WWII the governments of most of Europe decided they needed food more than a plush ride around town and dictated for most preexisting heavy industry to produce farm equipment. Similar to how companies here changed for the war effort Chrysler made tanks. GM made howitzers, etc. Some of the fancier companies in Europe went as far as burning documents and even assembly plants to hide this (embarrassing) use of their names. Even with Germany filming every stupid thing they did VW denied for years they provided support to the Third Reich. I think (not often) Audi helped out too. Unlike America, who held the Farmer up as a hero (then, not now) Europeans felt it a lowly profession and few companies wanted it remembered as such. Weird isn't it? Ford still boasts its agricultural roots (Henry Ford grew up on a poor farm and his whole drive was to build farm equipment.
 
Synthetic makes sense if you live in a polar climate. In Yellowknife synthetic flows instantly, even at 70 below zero. But with dino oil you listen to the lifters for a minute before the oil flows to the head.