I have found that if you can go 5000 miles on conventional oil, you can safely go 7500 miles on a synthetic. Past that, things get risky. The reason for the extra miles is that synthetic oil tolerates heat better. Heat is the main factor in the deterioration of oil and the most damage is done in the ten minutes or so after you shut down the engine. A thin film of oil is trapped in the engine and it must absorb all the heat from the engine as it cools down. It does not have oil constantly flowing to draw the heat away like it does when the engine is running.
That only affects a tiny amount of oil and on the next start, it gets flushed away with fresh oil, but it mixes with all the oil in the pan. After a period of time, more and more of this trapped oil is getting mixed in with the oil in the engine. This trapped oil does weaken the remaining oil a little, but it is cumulative. Eventually the oil in the engine can’t tolerate the heat as well as it could when new because of the slightly broken down oil mixed in. Now it begins to suffer more on each cycle and break down even more. If allowed to go too long, it begins to thicken into a substance called sludge.
If the sludge is allowed to build up, it restricts oil flow even further causing more damage to the remaining oil to the point that it can crystallize forming coke. Coke is a gritty hard form of carbon and it will really damage your engine.
But you just need to change the oil before it breaks down tot he point of doing any damage and for most synthetics in most engines, that is around 7500 miles.
In many “independent tests” that you can see on YouTube where people film their own tests, they will rank oils based on their criteria. Royal Purple usually does pretty good on these test, especially in the wear tests. If it really is as good as these tests indicate and it does have less wear, then theoretically your engine would last longer and that is where the value would come from, But that value is lessened if you start using it in an older vehicle which may already be near the end of its life.
It would also be of less value to a new car buyer who does not plan on keeping the vehicle for as long as possible. If you aren’t going for the million mile club or something like that, then any oil that will get you as far as you need it to is all you need. For most of us, all the major brand oils that are API certified will do.