Using 5W-40 on Z's with no oil cooler

Geoff-AU

Factory spec is safest. Nissan will have a bunch of lubrication engineers who have settled on the "most correct" viscosity for a wide range of reasons. But as Spooler said there are also regulatory hoops to jump through so "reasons" doesn't just mean driving conditions. If a 5W20 oil and a 5W40 oil both work fine but emissions is better with the former, then 5W20 or 5W30 will be specified and 5W40 will get dismissed.

Look at 2011Nismo's graph a couple of posts up. The VVEL system has to work with 5W30 oil that has a viscosity of 50 at 40 degrees C. At 0 degrees C the viscosity will be 100+ and the VVEL still has to function (at least under low engine stress, to idle the car and drive gently until oil temperature improves). I could fill my car with 5W30 and flog it as soon as the oil temp needle has started to come up, at 80C 5W30 oil has the same viscosity as 5W40 does at 100C.

There is some truth in "thicker oil provides more protection at high temperature" and "thinner oil flows better so it removes heat well" and so on. But unless you are building a 1000hp beast or spend most of your time above 5000RPM it matters a lot less than you think. I used to spend a good half hour staring at the oil aisle before making a decision but the fact my engine didn't blow up doesn't mean fussing over it that much was justified