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Fuels & Lubricants Discussion all about Fuels & Lubricants. synthetic oil, conventional oil, brands, change intervals, diesel grades, gelling and such debated items like that. Non TDI related postings will be moved or removed. This forum is NOT for the discussion of biodiesel and other alternative fuels.

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Old July 12th, 2017, 14:56   #1
New Mickey
The user formerly known as mickey
Join Date: Jun 2017
Location: Utah
TDI(s): 2015 Passat
Default AAA independent testing of synthetic vs. conventional oil

AAA has confirmed what high performance engine manufacturers, including jet engine makers, have known for decades: Synthetic is better. Way better.

Personally, I won't put conventional oil in my lawnmower.

The only application that I now of where synthetic oil is specifically DISAPPROVED is in piston general aviation aircraft engines. Companies like Lycoming and Continental forbid the use of synthetic oil, though semi-synthetics are approved in some applications. Why? I've been trying to find out for years, and the closest I've come to a definitive answer is that synthetic oil doesn't get along with the high levels of tetraethyl lead that's still found in aviation fuel. (Though I find the explanation a bit weak.) Personally, I lean toward the theory that pilots are just extremely conservative.

One of the benefits of synthetic oil that's often overlooked is that it has much better viscosity stability across a wide temperature range than conventional oil, and thus needs considerably less Viscosity Index Improver. For example, a 5w30 synthetic doesn't need nearly as much of that chemical additive as a 5w30 conventional. The Index Improver chemical does nothing to lubricate your engine, so the more actual oil it displaces the less lubrication you get. Worse, the chemical itself breaks down under heat and stress and causes harmful engine deposits. You can find a very wide "viscosity range" in a synthetic oil with relatively little additive to make it work, where a similarly "wide range" in a conventional oil would be chock full of that stuff. Not good.

Anyway, here is a link to the AAA's results:


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Old July 12th, 2017, 15:07   #2
New Mickey
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Here's an interesting "why not in airplanes" article that cites original research, and yet is less dogmatic than other things I've read.

Back in the 1960s Shell did experiments using all-PAO synthetic oil in piston aircraft engines. In LARGE engines the PAO did poorly at keeping lead salts in suspension, resulting in grey sludge in the ring belts and prop controllers. (Lead levels in aviation fuel back then were VASTLY higher than they are today, though.)

A lot more testing since, by a lot of different people for a lot of different reasons, shows that synthetic oil simply has very little effect on the wear in an aviation engine. This appears to contradict what the AAA found, but does it really? Airplane engines are highly over-engineered and under-stressed, for obvious reasons. (You can't park your broken down airplane by the side of a cloud.) They simply do not work very hard. You only use 100% power for a couple of minutes on takeoff, and that power level is surprisingly puny for the displacement of the engine. A Lycoming O-360 produces just 180 hp these days from 360 cubic inches. Embarrassing. And that's during takeoff. Other than that they're just loping along. So the wear rate is going to be negligible regardless of your choice of oil. Also, you're not going to increase the time between oil changes in your airplane, unless you're completely nuts. The manual says X number of hours, and that's it. The oil will get dirty just as quickly whether it's synthetic or not. (A lot of aviation engines don't even have oil filters, and get the oil changed every 25 hours of operation. That makes synthetic oil a VERY expensive proposition.) If your o-360 has a TBO of 2000 hours that would be the equivalent of just 120,000 miles in a car that averages 60 mph....and THAT in turn translates to a lightly driven car that spends all its time on the freeway. You could horse manure in your car's sump for that much time and it would survive.

Personally, I don't think that today's 100LL aviation fuel is going to be incompatible with synthetic in today's itty-bitty engines. If I had a turbocharged airplane I'd at least use semi-synthetic, or if I had a homebuilt using an automotive conversion. (A Viking, based on a Honda Fit engine, is on my short list.) You don't see a lot of enormous Pratt and Whitney radials out there these days, nor do you see crazy high lead concentrations in the fuel. The biggest argument against using synthetic in an aviation engine is that it's simply not cost-effective.

You run a car engine until it wears out. You run an airplane engine until the Hobbs meter says it's time to rebuild it. X number of hours, period. At which point it's actually nowhere near "worn out". So why waste synthetic oil in it?

Cars are a whole other matter. The oil is under far more stress, and you are trying to maximize the engine's life. You run it until it dies.



Last edited by New Mickey; July 12th, 2017 at 15:20.
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Old July 12th, 2017, 17:26   #3
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A number of years ago a member in the Ontario forum had his 100% synthetic tested at 8,000km, 16,000km, 24,000km and lastly at 32,000km. The lab pronounced the oil still very good at the last test.
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Old July 12th, 2017, 17:45   #4
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With a bypass filter and high quality synthetic oil like amsoil, you could probably get 50k miles before the oil needed changing. Won't be me though. I wouldn't want my golf to stall out mid flight.
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