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Originally Posted by aomdmm
Why non-synthetic if they both work about the same??
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Because its cheaper. I'll try to explain it, but I'm a picture person and not as good with words. Both oils contain similar additives making them have predictable qualities for wear, pH, and shear. The synthetic oil is purer, meaning it can dissolve more stuff. How much more stuff is a crap shoot. Based on testing by Blackstone Labs, the average safe lifespan of a typical dino oil is 2000-9000 miles depending on many factors. The average safe lifespan of synthetic oil was 2000-13,000. So depending on what your engine is giving the oil, synthetic doesn't last any longer.
The other thing to keep in mind is that its the same oil molecule. One was created in a lab, the other from millions of years of rotting dinosaurs. They both lubricate the same. One doesn't lube better than the other.
So, what I'm saying is that unless you test you own personal oil, there is no way of knowing how long it will be effective in your engine. I therefore change my oil every 3000 regardless of what type of oil it is.
Having said that, they both perform the same up until they exhaust their additive package. Typically, but not always, synthetics maintain stability for longer periods of time. Since you can't tell where that point is, using synthetic oil doesn't make sense to me. You can gamble and extend your oil changes with synthetic, but you'd better know what the additive package is and how your engine affects it. Since you also don't intrinsically know how many miles are city, or dusty, or damp, or highway, or cold... you can't accurately predict when the oil will need to be changed. Hence the 3000-mile interval. I'm sure it can go longer, but do you want to gamble that?