This may set off a firestorm but I would really like some hard data and reall-life experiences from people who have used full synthetics (particularly Mobil 1) and hear what the mechanics who have torn down engines that used synthetic oil vs engines that did not. Is a synthetic oil worth the minor extra cost in protection over the long-haul. And is it better in cold statrs and cold weather?
If you go into the archives you will find several dozen posts on this subject. In short:
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For normal driving conditions in a moderate climate, in a NON-TURBO US or Japanese car, any normal “dino” oil will do and will let your engine live a very long life. In other words, you’re wasting money with synthetic.
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If you tow a trailer, drive very fast, live in a very cold or very hot areas, the stability of synthetic will allow your car to start easy and live long as well. Cold starts at -35/-40 with a 0W30 synthetic is easy, and will not cause excessive engine wear. This type of operation with 5W30 or 10W30 regular oil will SEVERELY shorten engine life.
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If you own a German car, especially one with a turbo, you will likely HAVE to use synthetic since most specify it.
Although the above is a summary of benefits (and longer drain interval is NOT one of them) you should still go through all the past threads.
How about some hard data and real life experience from prople who have owned cars for over 40 years, in cold climates, some for hundreds of thousands of miles, and never used synthetic…and never worn an engine out?
You’re right…this subject creates lengthy threads.
PostScript: The guys are right. If your owner’s manual specifies synthetic, as turbocharged engines usually do, then that’s what you should use. My comment was really only for those cars that require only dino oil.
I use Mobil 1 in my turbo Mazda because of the turbo, and regular oil in the rest of my vehicles. I do not deviate what the owners manual states for oil change intervals in any of the vehicles.
I look at synthetic oils as application specific. That means if the conditions of operation of the vehicle can benefit from the advantages of synthetic oil, then it should be used. And here the conditions.
The vehicle is operated in extreme cold/hot conditions.
The engine is turbo charged.
The vehicle does heavy towing.
The vehicle is raced.
Only one of these conditions applies to one of my vehicles.
Tester
There are too many other variables to say an engine run with synthetic is better torn down than dino or vice versa.
It does help with cold starts in my experience. Over a cold spell(10F starts) I changed my oil from synthetic to dino and noticed a large difference. But in each case it started.
I own a turbo car (Subaru WRX) but just change the oil every 3k-4k miles usually with dino.
The car manufacturer has done a lot of testing. If they specify synthetic, you would be a real fool not to use it. On the other hand if they don’t specify it, then using it is likely to only cost you a little more for the oil and you might even get a little longer life from the engine (300,010 miles verses 300,000 miles. 
Goldwing48, I’d also like to see the hard data that you want but I don’t believe that people here have the resources to establish or have access to that data.
I’d like to know, from an engine designer or engine tester, why our 08 and 09 non-turbo GM cars with OHC engines, and with aluminum blocks having steel cylinder liners specify synthetic oil in the owner’s manual as I have a stock of non-synthetic that I’d like to use. It is permissible to add non-synthetic 5W-30 oil with the starburst on the label if synthetic is not available but synthetic oil must be used for oil changes. I have been following Car Talk for a few years; have also looked at BobIsTheOilGuy.com.
My experience. Started using Mobil 1 in my 2004 Mazda6 3.0l V6.
Changed oil at least every 5k, despite mfg recommendation of 7500. Engine broke down at 81k. Probably not the oil’s fault, but there you go.
I couldn’t tell the difference in the cold. I have nothing but inconclusive anything. The Toyota would start at -40F the Chevy 350 would not. I had a truck with Mobil one in it and it didn’t start any better in the cold.
I have owned cars since 1973, 85 of them. The only engine that fell apart was the one I ran dry of oil. The oil pressure sensor was leaking and it was disconnected. I figure the previous owner thought that disconnecting it would stop the leak.
In conclusion, it’s no contest. Truly, no contest. No data.
I switched to using synthetic over 10 years ago.
My 90 Pathfinder went through it’s entire life on regular dyno oil. I switched to synthetic after I had about 50k on my 98 pathfinder.
The 90 Pathfinder started burning a little oil after about 250k miles. When I sold it it had over 300k miles and was burning about a quart every 3k miles.
The 98 Pathfinder still isn’t burning ONE drop of oil between the 5k mile oil changes and it has over 350k miles.
I’ll admit it doesn’t really mean that much since the 90 Pathfinder while still burning a little oil was still running GREAT…even after 300k miles.
I cannot speak to relative wear, but I do have an anecdote regarding leaks.
I used Castrol 20W-50 GTX for 30 years in cars and motorcycles. No problems at all. Three cars went over 250k miles each. Then I got a a couple of 5 qt jugs of 20W-50 that looked like 90W. I tried it in one BMW and the Volvo and they both sounded awful on cold start. I sent a sample in to BP (owner of Castrol) and they tested it and reported that it was “within blending specifications”. I asked if their specification is simply “will pour out of the jug”.
Switched to Mobile 1. Problem - I cannot keep it in the engines. I have since changed valve cover gaskets in both '97 328 BMWs. Changed valve cover gasket twice in the '91 Volvo. All three still leak. One of the '97s leaks badly. (yes, I cleaned the crankcase vents) Recently the wife came in complaining that her 2004 BMW smokes and stinks from the engine compartment. You guessed it - leaking valve cover gasket.
I posted this on a BMW forum and another fellow reported the same experience switching from long-term use of conventional GTX to Mobile 1.
I’m changing brands again, and the older cars are going back to conventional oil.
20-50 wouldn’t leak out a hole the size of my thumb. There are probably leaks everywhere, and the thinner oil (is there even 20-50 Mobil 1?) comes out.
It would help if we knew what kind of application you are asking about. If you are asking about a Honda Goldwing (going my your user name), I believe Honda recommends synthetic oil for it. If I owned a $20,000 motorcycle, I would follow that recommendation.
The synthetic vs. conventional oil question isn’t a “one size fits all” question, so you won’t find a satisfactory “one size fits all” answer.
Real unbiased scientific data is going to be hard to find. You would have to sift through a lot of anecdotal stories and marketing hype in your search for real data.
I have a generator with an 8 horsepower engine that calls for synthetic oil if the engine is going to use the same oil, but be operated under extremely hot and extremely cold condtions. If the engine is going to be used under the high temperature extreme only, then 30 weight regular oil is fine. If it is going to run under extremely cold conditions only, then 5W-20 is called for. I keep the synthetic oil in the engine since I never know when I’ll need the generator. In extremely cold weather, I need it to power the controls and the blower motor on the gas furnace. In very hot weather, I need it to keep the freezer going. Strangely enough, the generator has seen more use in hot weather. In cold weather when our city had a bad ice storm, our power was restored very quickly since we are on the same major trunk as a Walmart and Lowe’s. On the other hand, in July when we have had heavy rains, we have an underground feeder that affects only 6 houses. I have been without power for three days until the power company switches us to another loop (a 5 minute operation) until they can dig up a part of the line and correct the fault.