This is definitely a touchy, and passionate topic, but no one ever mentions oil sampling. No matter the type of oil, heat and the byproducts of combustion are the biggest enemies of oil longevity. Chemistry resists heat breakdown, and precise fuel metering (modern fuel injection) limits oil contamination. Oil sampling will tell you the condition of the oil by seeing what impurities are in it. There are now lube-for-life diesel engines with super filtration systems (3 and 4 filters). The fuel injection and lube filtration is so precise that the oil stays clean. You only add to the level and there is no drain plug. I understand Duetz builds one like this and supposedly they claim 5000 hr emissions compliance. It's a roughly 30 city miles to 1 hour conversion, in forklifts..
Synthetic oils are superior in their ability to resist lubricity package (additives) breakdown, because of the package chemical compositions. The oil itself is merely a carrier for these additives and is the same oil used in conventional oils.
Back to sampling, a technical trainer at the tech support and training facility where I work did an experiment with a wrecked Ford fusion he rebuilt. It had approx 15000 miles at the time. He drove approx 100 miles daily on highway.
He changed the oil at time of finishing repair (no engine work done), then changed again at 500 miles using Mobil 1 5w20 and sampled. At 20k he sampled oil and changed filter only (motorcraft). 25k did the same, 30 the same and so on. As time went on the samples came back that the dirt (silica) levels actually got lower. And metals stayed about the same. At 55k he finally noticed a rise in metals and decided to change the oil although the chemists recommended resampling at normal interval. He figures he could have gotten another 10k before the oil would be chemically "worn out".
This i think works on an engine from new or newer designs. A worn engine or older design will never have the tight tolerances that new engines have. Our first gen Explorers will probably never achieve this but I think 20k on synthetic oil is realistic without harm. My dads 99 VW jetta diesel TDI oil change recommendation with 5w30 synthetic is 10k. At 280,000 miles, its still going strong, and returns about 50 mpg.
As soon as I have a new engine to experiment with i will try the same and see what happens. My problem is I don't drive any one vehicle enough to accrue the miles that fast.
Personally I run mobile conventional 10w30 in everything I own and change it every two years or about 5k, except where different weight is recommended. This oil is cheap, I get it for about $3 a qt through work.
P.S. my carbureted vehicles get oil change once a year or 3000 mi.
