Ding ding ding. A little added info.
Synthetic oils are not automatically better. The synthetic base oil has better oxidative stability and better low temperature properties than most mineral oils. For other properties, such as wear protection, cleanliness, friction, and corrosion protection - it's the additives blended into the oil. And since diesel oils have 2X the additives, any diesel oil will have more performane than any passenger car oil.
Round 2. There are now so highly refined mineral oils that they can match synthetics for high and low temperature performance. Recent "rulings" by the National Advertising Division of the Better Business Bureau (Castrol v. Mobil) said that these "Group III" base oils can be called "synthetic" in advertising. So yes, Castrol's synthetic uses a mineral-oil derived base oil, while Mobil is still using the (more expensive to make) synthetically derived base oil. In either case, you still need additives to formulate the all-around good performance.
Round 3. Do not underestimate the power of top-ups. Some long drain oils insist on huge 3-rolls of toilet paper filters, and although you don't change the oil, you change the filter every xxxx miles. Well, change out 3 quarts every once in a while and the "oil" can go pretty long, like never needing a "change." It's exceptionally hard to conduct a fair test, but comparing oils while topping up is changing the rules as far as a technical comparison.
Round 3. Viscosity is a key factor, but is almost designed to change during the drain interval. The additives that thicken the oil shear quickly and so the first 100-1000 miles will thin the oil from new. Then as contaminants build up or oxidation occurs, the oil will thicken. So while it may stay within the viscosity grade upper and lower limits, it will move around. It's normal and part of the formulation game. Having a very shear stable viscosity improver and a very oxidatively stable base oil is more cost.
Round 4. All oils are formulated to the same performance target, so to point out that one oil has more magnesium and less calcium etc, just says that one soft drink uses this sweetener, the other uses another. They're both sweet.
Round 5. Copper is also often found when it is caused by corrosion. Fittings, coolers, etc, may "rust" (or whatever copper rust is called.)
Round 6. One cannot rely automatically on TBN for oil change intervals. By monitoring used oils one can see what does give out first, but it is not automatically TBN. TBN neutralizes acids from combustion or fuel by-products, so perhaps high sulfur fuel or less than great ring sealing would cause TBN to deplete more quickly. Lots of low temperature operation would deplete the dispersants more quickly. Lots of high temperature operation would deplete the anti-wear additive (because it does double-duty as the anti-oxidant.) The answer to when to change your oil is "it depends."
Round 7. New oil isn't worse than used oil. Sorry, it just isn't so.
Round 8. The reason there is so much "controversy" over oil change intervals is that it's the fluid that gets stuff delivered to it. Gear oils, transmission fluids, coolants, brake fluid all are in a relatively "closed" environment and so they are often fill-for-life. Engine oil has fuel, water, dust, fuel additives, and maybe coolant added to it, so it has to defend itself. And that is the root cause of why it's hard to predict and dangerous to assume an oil change interval. You might consider changing oil every 300 gallons of fuel consumed. You might change it less often on highway trips. You shouldn't extend drains just because you use a synthetic oil. Truck fleets religiously use oil analysis to determine drain lengths, but can do so only because the truck is on the same route, same distance, same oil every time. It starts once a day and is running warmed up all day. How many passenger cars have that routine?
Round 9 Sure, the 3500 mile OEM recommendation is conservative. They don't pay for the oil. They do pay for warranty work. OEMs hammer on oil companies to make better oils. They don't buy them. Consumers pay for them. OEMs pay for warranty work. Do OEMs extend the drain allowed if you use the new oils? No, since they pay for warranty work. It will be a while before the car companies advocate extended drain.
Round 10 So if you can't accurately predict a longer oil drain interval and you're not going to get help from the OEM, what to do? Change when recommended. Or add protection. 90% of the added benefit comes from additives blended into the oil, and diesel oils have more than twice the additive level as passenger car oils.
Post-fight hint: A good oil turns dark quickly! The dispersants pick up sludge and dirt and suspend it in the oil so it is removed with the next oil change. A poor oil may stay cleaner-looking longer. So used oil analysis is indeed the way to see what is happening.
I'm sure I didn't answer everything, maybe added more to the fog. It's a pretty interesting and complicated topic.
