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Originally Posted by Chainlink
From my understanding, synthetics dont break down or seperate like conventional oil (including slightly contaminated) and this is one of the main reasons to its 15000 or 1 year in engine shelf life. A mechanic I trust told me the biggest reason to run it isnt the viscosity from the intial oil change but its abilty not to break down with contaminates. Hence viscosity stays good through out your planned intervals(with in reason). He did add it will eventually break down but it takes alot of wear and contamination. But we are comparing crude to full science engineered.
I switched to Castrol full syn when it came out with no regrets whats so ever in all my yotas even one with over 200k on the odometer and it loved it. I change my oil every 7500 and it looks like I just changed it. My wifes rig has mobil one and seems like the results are the same( but for me nothing but Castrol  ). |
Both synthetic and crude-based oil is changed primarily because the additives are depleted and the oil contains suspended contamination, not usually because the base lubricant is broken down (yet). But, all oil will eventually break down. It's because the additives have been all used up and the oil then quickly begins to oxidize at the boundary state. This is not unique to crude oil, all oil would break down without all the additives. FWIW, don't think for a minute that something like a GTX is much less engineered than a Mobil1. The difference is that base lubricant in a synthetic comes from a non-mineral base that is synthesized from typically paraffin. After that, the two oils are usually more similar than different. Now the base oil IS very important, but the modern oils that come from refined crude are really quite advanced and will do a fine job lubricating your engine.
While the marketing says you can extend the change interval, it's not a good idea to do so without at least doing a used oil analysis to see what's going on. I found switching from GTX to Syntec that my interval could not increase and the switch to Mobil1 only allows a small increase. At first, this seems wrong, but both GTX and Syntec are Castrol and so probably share similar additive formulas. In all cases, the oil will rely on additives under extreme conditions where the oil is no longer in its regular lube state, but where it experiences heavy shear force (i.e. like when it's squeezed going through the oil pump or on rod/crank interfaces), to cushion the two wear surfaces. So the additives in GTX and Syntec are probably similar and why I could not get longer service life. Might be that since Syntec is not a PAO-based oil that it's shear is not that much better than GTX, who knows? I do know that in my engine it is barely better.
Still, I follow at most a 4,000 mile interval between changes on Mobil1. My engine is 179K mile old and most of my analysis showed combustion byproducts (mostly carbon and some ash), dissolved fuel and dirt were contaminating the oil. The wear numbers (oxidation and viscosity of the oil itself and suspended engine metal) didn't look bad and those would indicate a slightly longer interval is OK (although I don't suspect a massively longer interval with the fuel and ash levels). It also helped a lot when I threw away the K&N filter and went back to a factory filter, the silicon numbers went down significantly. Anyway, before leaving oil in beyond what the factory calls for, check the oil itself and see if it's wise to do so. Even the factory specified interval might be too long depending on lots of things. If you're driving in the desert with lots of dust and sand, then I certainly would never leave an oil in for 7,500 miles. But a relatively mostly pavement with a new engine, broken in well with a good, clean intake system could certainly go 10,000 miles on good oil.