Originally Posted By: Shannow
Originally Posted By: CATERHAM
So yes if you're running continuously for months on end in a cold climate you can certainly benefit from running a 0W-20. Of course having oil gauges is best to monitor things if you're going to deviate markedly from the lightest oil specified by the manufacturer.
But my point is that this the requires everyone who is operating the car to understand the limitations of what's in the sump...and not take it on the emergency funeral run without doing an oil change.
My Caprice has a big winged alloy sump, and still hits 105C oil temperatures (type K thermocouple dropped down the dipstick) when it's freezing, in as little as 20km on the highway. (before you start, that's a 3.2cst HTHS 5W30)
As an example of my argument, I'll use something that you are familiar with, the owner's manual of the Nissan 510.
Clearly, 5W20 was acceptable (not a recommendation) at continuous operation at ridiculously low maximum temperatures. It's similar to your assertion in this thread.
SAE40 was acceptable (not a recommendation) at continuous operation at ridiculously high temperatures...reverse argument to yours, but still applicable.
But the Goldilocks recommendation (and I say that because it's at the top of the list and covers all of the temperature ranges) was 10W30, which rather than being you "belt and braces", was just more suitable for "normal" people, doing normal stuff with their cars, all year round.
I thought my explanation of operational viscosity was pretty clear.
To repeat, those old viscosity/ambient temperature charts are a very crude way of telling the operational viscosity story without any reference to oil temp's or oil pressure.
If you know your oil temp's then ambient temp's don't matter once you know the safe maximum oil temp's for an oil grade but oil pressure is the real bottom line. Once you know what the established safe minimum value is at some elevated reverenced rpm you're in total control.
Fast forward to the present time when generally just one oil grade is recommended for year round use. If that recommended grade is say an A3/B4 5W-30, in an OP and OT equipped vehicle on that spec' oil, the owner will be able to establish under all driving and climatic conditions just how much viscosity reserve is present. With that knowledge one may come to the conclusion that running something lighter might still allow a more than adequate viscosity reserve year round or perhaps just during the winter months.
Once one is totally familiar with the OP and OT of their car there really is no risk in running an oil that is too light, on the contrary if something out of the ordinary happens that could significantly lower the operational viscosity (oil pressure), such as a leaking fuel injector causing fuel dilution, you'll know it immediately and can take remedial action. If it's unusually high oil temp's as a result of trailer towing in a heat wave, you'll know it and can modify you're driving behaviour accordingly.
The point is you're in control of how much viscosity reserve you're comfortable with.