Traditional dino products are a blend of components. Over-simplified, mix some thin components with some thick ones to arrive at a 'medium'. The more volatile of them are short molecule 'chains' that also break down more easily in normal use and combine with impurities (from that breakdown) to create the dark & dirty look. This 'weakness' is why so many such traditional dino 'blends' have a shelf life.
For an experiment, put a quart of gasoline (a hydrocarbon blend) into a mild jug and cap it. After a while, you may smell ether (lightest) that permeates the plastic jug, not much different than CO2 that dissipates from an unopened 2 liter soda bottle in a year or two. The remaining dino components becomes thick & gooey at length from what
doesn't evaporate or dissipate as easily.
Road *tar: another 'blend' of (heavier) hydrocarbons. That it fades and stiffens (surface) with age is the same 'loss' of more volatile components. (*.. what refinery 'leftovers' become)
Synthetic oils are blended from more similar-sized (length) molecular chains than dino and are thus more stable against contamination and 'breakdown'. The less dirt apparent in drained syn oil is often just less of the oils own 'broken' molecules and semi-soluble contaminants that attach to them. If you
make less pollution, neither drained oil or the filter will show as much. There should be no confusion as to whether synthetic oil is indeed better.
Of course
is, but
whether its cost is justified will always be a case by case matter of opinion and confidence. I suggest that low hr/mi use favors the economy of cheaper dino, as it is refreshed more often and the crap rinsed out. ex: Since I don't put 3k mi/yr on either car of the truck I'd be well of to switch to synthetic so my oil doesn't 'sour' between seasons and service intervals.
Few things are more YMMV than which is best for a given use. Cost wise, it's not much different than considering change intervals, and that adds 'V' to 'YM'.
