For a lot of people, the first good or bad experience defines if using a product is positive or negative.
Before "full synthetic" meant containing mostly visom or severely hydrotreated base oils (ie. Group III), synthetic oils would typically contain 70 or more percent PAO (Group IV) and a fairly good dosage of esters (Group V). Oil companies were selling gas mileage, extreme weather, performance and longetivity advantages by going to synthetic motor oil, promoting all the reasons it's better than conventional. Unsuspecting folks who had already put significant mileage on their middle-aged and older vehicles sometimes would think they ought to give a synthetic a try. What they did not realize was the seals and gaskets in their engine were in poor condition and the only thing preventing leaks was all the varnish, resin and sludge that was gumming up the holes. So after running the high percentage PAO synthetic (which also shrinks seals and gaskets) for a few weeks, the synthetic oil would dissolve all the junk holding the gaskets together and there would be oil running out of every seal and gasket on the engine and a big puddle on their driveway. To people who have a hard time thinking through their strategy to the next paycheck, synthetic oil was a problematic useless scheme that oil companies concocted to get a higher price per can/ bottle.
Several older mechanics I know generalize, "just because it's good for a few days in a racing event in a race car doesn't make it worthwhile for street driven vehicles." With that one bad experience or perhaps an account of it from another driver, they have not realized that seal and gasket materials have significantly improved, making them more resistant to synthetic attack, and - the vast majority of synthetic base stock is no longer Group IV or V, it's III - which has very little seal attack properties by comparison, and it is actually cheaper in the long run to run synthetic because extended oil changes will reduce the cost per mile in oil expenses to less than that for conventional oil. And further - even the extra expensive "real" (German-approved) synthetics carry seal-sweller chemicals which prevent PAO seal shrinking.
Probably a case of someone who was once burned by a hot stove who now tells everyone to stay away from it.
