Sure there is. When you drain it, you can tell. Stick your little finger up into the hole and feel around. If you feel some sludge, it was conventional. If its smooth, it was synthetic.
I own a pennzoil oil change center. I am in GA, so it wouldn't benefit me to sell you something, as chances are you'll never buy from me anyway. (although, stranger things have happened)
Here is the real lowdown on the difference.
Conventional oil begins to break down after it get hot and beaten. You see those thermal viscosity commercials that Castrol used to do. They were full of crap. You never could read the fine print, but when recorded and read, they were comparing their best synthetic to a leading conventional. Con artist and nothing more. As the conventional oil breaks down, it turns to a tar looking substance, and develops a "sludge" for lack of better terms. That tar like substance hardens on the outside, which will cause blockages in oil passageways. (kinda like cholestoral in arteries) Sooner or later, if abused enough, it will finally completely clog and the engine will starve itself of oil at one major point or another, leading to failure.
Synthetic Oil doesn't break down. What happens to synthetic is that the additives that make it work wear out. Over time, it looses lubricity (yes that is a word) In essence, instead of functioning like oil, it begins to function like a simple lubricant. When the synthetic breaks down far enough, it will become like watery crap and again, failure will be likely.
Don't ever believe in marketing. Marketing is simply a tool, and fools believe everything they see and hear. Lots of people ask me, why should I change my oil at 3000 miles when the dealer says 7500. Would you go to the dentist for a broken arm? Auto manufacturers make cars, not oil. When an oil manufacturer says do it at 3000 miles, it best to follow their advice. And in most vehicle maintenece guides, it says for severe service change every 3-5 thousand miles. Severe service is defined as
stop and go driving, long haul high speed, dusty condition, extreme hot or cold, and so on. Amsoil is the biggest joke on earth, and unfortunately Mobil has decided to follow suit with their 15000 mile oil. I have done this for 10 years, and I don't believe it for a second. Do you think Mobil is going to pay you for a blown engine? Not likely, about 1 in a million.
For your expedition, I would run a 10/30 synthetic, or synthetic blend oil. THere is also a high mileage oil that will benefit you, as your expy will probably burn some oil. That oil will reduce the amount burned. I have 89000 on my navigator 5.4 and I run full synthetic 10/30 and it still burns 2 quarts in 3000 miles.
If you want to get further into that, just let me know. I have quite the wealth of oil knowledge, but it can get boring
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