Motor oil does not become thinner as the miles accumulate on it. It did back in the days of carburetors when we had cylinder dilution caused by the inefficiency if a carburetor. Today's cars have fuel injection systems and cylinder dilution is a thing of the past. Injection systems measure the fuel delivery so precisely that it simply doesn't exist. Motor oil actually becomes thicker as time and miles accumulate as the lightest molecules "boil off" in dino oils. Synthetic oil is "built" with uniform molecules and doesn't experience the "boil off".
Fuel dilution is definitely still a thing, there are multiple threads on it, in fact there's a thread adjacent to this one on it where a guy is complaining about 13% fuel in his Honda 1.5L engine, which is notorious for fuel dilution.
While high levels of fuel dilution were uncommon with port injected engines, particularly those that weren't high performance, those with forced induction or those with high compression and with performance tunes still experienced it. The S62 in my E39 M5 fuel diluted, the SRT engines all fuel dilute, the AMG and Porsche engines diluted, same with engines like the LS7.
With the introduction of direct injection, and turbo direct injection, fuel dilution in regular non-performance cars came back in a big way. Some engines are far worse than others, but it's an issue across the board. Some manufacturers have resorted to adding back port injection so that they have a hybrid injection system, running the port injection on warm-up and low speed/idle and switching to DI under demanding performance conditions where it shines.
Both synthetic and conventional oils are formulated in the same way, which is the selection of a blend of base oils, that, coupled with PPD's (pour point depressants) will be able to meet the anticipated performance requirements of the intended Winter rating (0W-xx, 5W-xx, 10W-xx...etc). Since synthetic base stocks have higher natural VI's and better cold temperature performance, they can start with a heavier base oil blend and will subsequently require less in the way of VII's (Viscosity Index Improvers, which are polymers that expand with heat to keep viscosity up, and contract with cold, to limit the rate of thickening as the oil cools) to meet the target viscosity.
Oils blended with PAO will require little to no PPD's to obtain their cold temperature performance and can thus be blended with heavier bases still and in many cases are capable of being blended into a multi-grade without any VII's at all. This includes 0w-16, 0w-20, 5w-20 and 10w-30, but is generally uncommon, though these grades, blended with PAO will have very low VII content.
The volatility of the lubricant is measured using a test called "Noack". Synthetic, conventional and blends are all subject to the same limits and there are some very volatile synthetic base oils as well.
In service, an oil has thee things working against it to change viscosity:
1. Fuel dilution, which we've already discussed, this reduces viscosity.
2. Shear of the VII polymers. This depends on the VII type used and engine design. Some designs have a higher propensity to shear oil than others. Certain VII types are significantly more shear resistant than others, but then require higher doses for the same effectiveness and are more expensive. This is a balancing act. Shear reduces viscosity.
3. Oxidation. Exposure to heat and depletion of the additive package (oils contain anti-oxidants) can cause the base oils to oxidize. Along with lighter fractions potentially flashing off can result in the viscosity of the final product increasing.
Blenders cognizant of the ongoing fuel dilution issues with direct injected engines may be balancing viscosity loss with viscosity gain via oxidation, using less anti-oxidants in the base oil blend to allow for some natural oxidative thickening to cancel out some of the viscosity loss due to fuel.