It might slightly improve fuel economy (especially in the winter), and it should actually decrease wear when the engine is cold and improve driveability in the winter. Why do I want to improve fuel economy? Mostly just because I can. It's kind of fun to see how high I can go making just minor changes. If I really cared about the cost I would get a more efficient vehicle. Oil alone probably won't make a measurable difference in fuel economy, but combined with other things like synthetic gear oil, beam style wiper blades, possible other aero mods, etc, it might add up to +1mpg.
Also, thinner oil decreases loss in the engine, therefore usable power at the wheels is increased (very slightly).
Toyota originally recommends 5W30, but there was a TSB a while back that approves 5W20 (not 0W20, but no one knows why). The first number means that it acts like 0W oil when the engine is cold, and the second number means it acts like 20W oil when it's warm.
So when the engine is warmed up, 0W20 is in the same viscosity range as Toyota approved 5W20. At 100C Mobil 1 0W20 is rated at 8.7cSt and 5w20 is rated at 8.9cSt, basically the same. 5W30 is rated at 11cSt, so *w20 is 20% less viscous, but it shouldn't matter because Toyota says that's OK for our engines.
When the engine is cold, all oil grades are thicker than what the engine needs at operating temperature. At 40C (a hot summer day), the oils have the following viscosity:
0w20: 44.8cSt
5w20: 49.8cSt
5w30: 61.7cSt
That's about 5 times what the engine is designed for (let's say it works best between 9-11). The "conventional wisdom" is that thicker oil protects better because it has a better film strength, or because it sticks to surfaces longer, or something like that. That may be true, but engines constantly pump oil onto surfaces that need it, and if it's too viscous there won't be enough oil flow. With cold oil, pressure will be high but flow will be low (think of sucking a milkshake through a straw). Thinner oil decreases the oil pressure (because it's easier for it to flow through the engine) and increases flow. That's fine as long as the oil pump can still maintain enough pressure to keep oil flowing to where it needs to go.
You can read more at BobIsTheOilGuy (
Motor Oil 101 - Bob is the Oil Guy - Bob is the Oil Guy), but I've summarized most of his Oil University series here in a couple paragraphs.
So how can I be sure I'm not damaging my engine? Mainly the fact that 0W20 is not that much different than 5W20 that Toyota says is OK. Ideally I would have an oil pressure gauge to experiment with different oils, but our engines just have a dummy light for oil pressure, and it's probably not worth it to add a gauge. So I won't know about oil pressure unless it's dangerously low, but I don't expect it to be a problem. I can also have a used oil analysis (UOA) done. UOA is kind of pricey at $25 for a sample, but if it saves the work, expense, and waste of doing an oil change before it's needed, it could be worth it. UOA can tell the condition of the oil that's in the engine, and it can also give some insight into the health of the engine based on what metals and contaminants are floating in the oil. If the engine is wearing unusually fast, it will show up in UOA.
So why don't car companies recommend 0w20 for everything? They do now! Not long ago, 0w20 was exotic and expensive. But now I can get it at Walmart for the same price as any other oil worth using. Even if synthetic is a bit more than conventional, I can probably run it for 10,000 miles without trouble (UOA will confirm), so I'll save time and money over synthetic. If mileage increases 2% (from 18 to a whopping 18.3), I'll save 11 gallons over 10,000 miles.
The 4.7L V8 is known to be easy on oil based on some UOAs I've read, so I don't expect to have any problems going to 10,000 miles. I'll get a UOA around 5-8,000 miles and decide based on what comes back. The analysis includes commentary from an expert, so you don't need to understand the details of it for it to be useful. Ideally I would get a baseline with what's in there now, but the cost adds up fast, so I'll skip it. I found at least one report of someone intentionally using 0w20 in a 4.7L (and at least one person who accidentally had it put in at the dealer), and no reports of a 4.7L blowing up as a result of 0w20, so there's some more evidence it will work.
It's likely that something else will give out long before the engine dies, anyway. I'm not concerned with whether it will last 300,000 miles or 500,000 miles because that will be 20-40 years from now if I drive 10,000 miles a year. I want a self-driving electric car by then.
