And if we really want to get technical, oil viscosity has everything to do with film thickness AND film strength which has a direction correlation to load support capabilities and wear / bearing life.
HOWEVER, higher viscosity also creates more pumping losses for the positive displacement gerotor oil pumps AND greater rotational resistance in the rod bearings AND increased heating of the fluid. So there's a range of viscosity that are optimal for a given bearing clearance, diameter and width.
Generally, with widely used clearances in the industry for rod bearings in most passenger cars and pickup trucks, 30 weight is right about the sweet spot at balancing protection vs. drag losses / heating which typically produces about 10cSt viscosity @ 100C. Then of course we don't want the oil to be too thick when cold due to pump pressure limits (which affects wear, fuel economy, power etc.), so we have more temp stable oils called multi-viscosity oils. 5W-30 means that the oil is a 30 weight at 100C (212F or boiling point of water), but has the equivalent viscosity of a straight 5 weight when at 0C (32F or freezing point of water). Even then, a 5 weight oil despite being 6x thinner than a 30 weight at the same temp is nearly 5x-6x thicker at 0C then the 30 weight is at 100C, aka, despite it being more temperature stable, it still gets very thick when it's really cold!
For reference most passenger car and truck rod bearings use about a 0.002" to 0.0015" clearance even for vehicles running 5W-20 or 0W-20. They generally use the same clearances as motors built for 5W-30 or 0W-30 motor oils. The entire reason for running a low viscosity oil is to reduce pumping losses (hence better fuel economy) and improved cold weather emissions, but at the expense of heat tolerance and wear during heavier loading conditions. Generally it nets about 0.5 mpg difference on average, which sounds meaningless, but when you make millions of vehicles it adds up.
And you can run a different viscosity than the manufacturer's OE recommendation, but I would not recommend doing so for non-performance cars unless you have a very specific reason and unsterstand how viscosity impacts other critical systems such as cam phasors, hydraulic lash adjusters, hydraulic timing chain tensioners etc. For example, too thick or too thin of an oil can negatively affect how fast cam phasors can change their phase angle which directly affects valve timing and consequently power, emissions and even reliability. Some systems are more sensitive to this and others. Just understand it before you do it, but for most of us and our typical applications, stick with a good quality and reputable motor oil in the recommended viscosity.
Further more, viscosity and consequently film strength and thickness is also highly dependent on the temperature the oil is operating at. If you run your 5W-30 at 240F instead (think Mustang V8, Corvette or other performance car running at sustained high rpm on the track or even hooning around on a hot day) of the common 212F, your 5W-30 produces the same film thickness as a 5W-20 operating 30F cooler. So not only does the oil's properties play a critical role, but so do driving dynamics, engine design and oil cooling capabilities (no oil cooler, water to oil cooler or air to oil cooler etc.). You wouldn't want to track a stock Mazda6 2.5T for example and sustain high RPM because you would over heat the oil rapidly and possibly cause catastrophic damage (ECU's have thermal protections built in, but they are not always 100% able to prevent severe damage caused by misuse).
Generally you can take off the equivalent of one hot grade for ever 20F to 25F higher temp. So increase oil temps by 20F and your 30 weight drops to 20 weight. Then we can throw in other dynamics that increase the thermal load of the motor oil like a water jacket cooled turbo charger and much higher average and peak loads in the rod bearings that TDI engines see relative to NA (naturally aspirated) counter parts and you can see how the oil thins more and is trying to simultaneously support greater loads, one my question pushing long OCI's on TDI engines, especially in SUV's, towing or frequent cargo hauling applications.
They simply generate more torque per a stroke and at a given RPM than non-turbo engines and have additional heat loads both in the bearings (due to high loading) and via the turbo bearings which are oil lubricated and oil cooled even though a significant portion of the turbo's waste heat is removed via the coolant (all modern factory turbo engines are both water and oil cooled, the housing has a water jacket and uses the engine coolant, the shaft bearings are cooled and lubricated by your motor oil).
Adding even more to it is that angular velocity also plays a role in film thickness and heating. Higher RPM supports a greater film thickness but also more heating of the fluid. BTW that's why you shouldn't floor it on a cold engine, because the oil is 5x or 6x thicker than when its hot, your oil pump has a bypass value to alleviate pressure if it gets too high. So your positive displacement pump is NO LONGER POSITIVE DISPLACEMENT due to the bleeder valve. When the oil is up to temp, the PDP never sees pressures high enough to bleed off oil flow, so the flow rate remains constant relative to a given RPM and pressure is what varies with RPM.
But when the oil is cold, the pump pressures get insanely high, so to prevent mechanical damage, a pressure relief valve prevents over pressure. However the flow rate is then less due to the bleeder valve, which can cause localized over heating of the oil in the bearings. That's one way to throw a rod bearing or cause scuffing on a cold engine. It's possible that many modern engines have some ECU protections build in, since the ECU (PCM or what ever other term the micro-controller system goes by) now controls air, fuel and timing, automotive designs can limit how much air it will actually allow through the throttle body, so you can't force a lean condition like in the days of drive by cable. It will simply ignore driver demand and say "this is all I can give you right now". But, do you really want to chance it when all you have to do is just drive normally instead of hot-roding around on a cold engine? But I digress....
So all of these new TDI engines generate MUCH higher torque loads at MUCH lower RPM which is very counter productive for bearing life. Granted modern bearing designs can support loads that older designs could only dream of, but stress is still stress, bearing designs have had to evolve along with other traits of design. Everything is being asked to do more.
This is just one more important reason to do at least one or two UOA's and look at ALL of the factors in your oil's performance, from additives to viscosity to alkalinity etc. If any one factor drops below threshold the oil's performance has been compromised to the point where increased wear, reliability or resistance to corrosion is likely and will more rapidly degrade engine performance. If you want it to last, keep it well oiled and clean.
That's going to be the difference between and engine that lasts 250k to 300k or one that starts to see severe mechanical issues around 100k to 150k. If it's a lease, you probably don't give a crap other than for contract purposes, but if your an owner / operator then it's a different story.
Moral of the story is you probably don't need constant UOA's, but spending a few bucks to figure out what the optimal service interval is for the oil you use, your particular car, environement and driving habbits is certainly beneficial. Then just stick with regular changes and good quality filters (OE is always a safe bet for filters, which is yet another whole ordeal and topic on it's own).