Oil dosnt thicken as the temps increase, but a 40 weight will be thicker at a given temp than a 30 weight. For severe suty, turbos, and high sustained temps, thicker oil eill give more protection than thinner oil.+1
Honda’s engineers specified either a 5w-30 or a 10w-30 oil for NSXs. When they designed the NSX’s engine, they sized the oil pump and the oil passages to flow a certain volume of oil at a certain pressure to the bearings, camshafts, etc. given a certain viscosity (thickness) of oil. If the oil is thicker than Honda’s engineers planned, you’ll develop good oil pressure but you’ll get less flow than was calculated. If the oil is thinner than planned, you’ll get a good volume of oil but you won’t develop the pressure at the bearings that was desired.
At normal operating temperature (100° C), a 5w-30 oil, a 10w-30, a 0w-30, and even a straight 30 weight oil all have a viscosity between 9.3 and 12.5 cSt. At normal operating temperature, all of those oils will protect your engine just as Honda intended - none of them are too thin.
At lower temperatures, all of those oils get thicker than 9.3 to 12.5 cSt. They’ll all give you good oil pressure but until the engine warms up, none of them will get you the volume of flow that Honda wanted. The straight 30 weight oil will thicken up the most and the 0w-30 oil will thicken up the least. At startup, the 0w-30 oil is the least bad for your engine.
At the other end of the spectrum, when the oil temperature goes above 100° C, as it does on the racetrack or the Autobahn, all of those oils will get thinner than 9.3 to 12.5 cSt. If you really push the engine you don’t want the oil to get too thin. A 10w-30 oil will tend to thin out less than a 5w-30 oil, which will tend to thin out less than a 0w-30 oil (especially over time). So a 10w-30 oil will tend to give you a bigger safety cushion than a 5w-30 when you’re really pushing it.
If the ambient temperatures aren’t too low at engine startup, Honda recommends sacrificing cold flow properties for a larger high-temperature safety cushion. However, if you don’t drive your car very hard, you probably don’t need that safety cushion. In that case, a 5w-30 oil will result in less overall wear than a 10w-30 because of the better startup protection (as long as you change your oil frequently).
I have an engine oil temperature gauge in my NSX. On the Autobahn, the temperature sometimes gets close to 150° C before I slow down. At those temperatures, oils thin out a lot so I need a relatively thick oil to give me good oil pressure. I’ve found that a Mobil 1 5w-50 works well but the drawback is that at normal operating temperature (100° C) it’s thicker than Honda’s engineers wanted. As a result, it gives me very good oil pressure at 100° C but if I had a stock oil pump, it wouldn’t give me as much flow to the bearings as Honda’s engineers wanted.
In summary, if you’re not going to drive the car very hard, I wouldn’t worry about putting a 5w-30 oil in it regardless what the ambient temperatures are. If you drive the car hard, a 10w-30 or perhaps an even thicker oil may be better, as stuntman mentioned. If you follow the recommendations in the Owner’s Manual and change your oil frequently, you’ll be fine. But if you think through how you use your car and what viscosity ratings mean, you may be able to find an even better oil for your car than the blanket recommendations given in the Owner’s Manual.
Also a 0W30, 5W30, 10W30, and straight 30 weight should all thin out the sameand have the same viscosity at operating and higher temps. Other than warmup, all should have similar operating and higher performance.