I worked as a master mechanic from about 1989 to 1997 and still work on my own cars, when I can. I started putting synthetic oil and other synthetic fluids in my own cars around 1991, and ever since. I can tell you that the cars I serviced that always ran synthetic and had regular oil changes (<=5,000 miles between changes) would often be spic and span inside the engine. Take the motor apart and the valvetrain, timing components, and lower end would all be bare metal clean. The cylinders even after 200–300k miles would often look like new, the factory machining (“crosshatch”) pattern still in the cylinders and no perceptible wear, and much reduced wear on the engine bearings. It got to the point that when I changed the synthetic on my water-cooled VW and Audi engines, if the drained oil wasn’t coming out clear after 3000–5000 miles, I knew something was wrong.
Compare that to the engines that ran on conventional oil would almost always (unless that oil were changed religiously <=3,000 miles, which is costly and wasteful) have varnish and buildup on the internal components and more wear. If someone wasn’t good about changing their conventional oil, you’d see motors with caked-on buildup to the point that the engine would fail, often catastrophically. Conventional oils break down and leave behind stubborn contaminants. So, yes, synthetic oil definitely makes a big difference. Ever since 1991, I have always run synthetic oil and have never had a mechanical engine failure (knock on wood!), even in high-performance engines and Italian cars that are run hard and on the track.
Volvo Maintenance Schedule