I'm from the era of 30w motor oil, when many drivers added a can of STP to boot.....
My new KIA engine came with "synthetic blend 5W30" I just don't understand.... Whenever I pull the dipstick, the oil looks as thin as water ! How can this stuff possibly protect/lube moving engine parts ?
My owners manual recommends 5W30, ( 10W30 in hot climates ) ( I think Southern Arizona would qualify )
Next oil change, I'm going back to good ol' Pennzoil 10W30.
As for synthetics, I tend to agree with the poster who wrote; "Unless you live in the snow-belt, you don't really need it"
One reason they want the 5W is fuel economy, it is theoretically better for tight tolerances too. I've been using it in my Hyundai 3.3 V-6 with no problems at all. It does not have to be thick to work properly. I'll take what the manufacturer recommends over your "I just don't understand....". I bet they've done more testing than you have.
Also you have to keep in mind that the first number is the (generally cold) temperature (in °C) that it reaches its pour point, and the second number is the viscosity at 100°C. So it feeling thin at room temperature doesn't tell you what it's going to be like at operating temperature.
IOW - both synth and non-synth are going to feel very thin at 100°C (ignoring that it's going to burn the crap out of you). A 5W-40 synth will feel thinner than a 10W-30 non-synth at ambient and colder temperatures, but be thicker at normal engine operating temperature. So it is closer to the ideal by being thinner than the non-synth at colder temps, and thicker than the non-synth at higher temperatures. The greater the extremes in temperature, the greater will that difference in viscosity be. Synths can be made to have greater spread between the two numbers with far fewer relatively unstable additives being added.
Used to be that making a wide spread on the numbers for a non-synth made for a very unstable and short lived oil (because the additives required to do that compromised other properties of the oil), but my understanding is that over the years, the viscosity extenders have been improved so that you can get a wider spread than you used to be able to. But it still won't beat a synthetic for combined long-life stability and a wide spread between the two numbers - is that your understanding, Ed?
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