Sorry Hoonz I'm not down with your logic
.
I agree that oil temp varies more than water temp; but the oil is still ultimately cooled by the cooling system (even without an oil cooler.).
I also agree that thicker oil will drain down slower/less than thinner oil, even at operating temp.
But your logic is off about the protection, which is "better" for the engine, etc.
Even an average 15W-50 drains off vertical faces a lot more than you'd expect when warm. Of course it leaves a slightly thicker film than a thinner oil, but it's not proportional to what you see when the oils are cold.
Even then thicker film still offers bugger-all lubrication to a running engine; of course it does offer more lubrication than a thinner film, but that's not the point.
The point is that most components rely on oil pressure for lubrication, not just the presence of oil.
The system is designed for a certain range of viscosity; to circulate the oil with appropriate flow and pressure, increase the viscosity and you're decreasing the flow and pressure.
So you might have a bit more lubrication for the first few revolutions immediately on start-up, but for the thousands of other revolutions while the oil warms up, you have less pressure, thus less lubrication.
Once warmed up, if the oil really is running slightly hotter than the engine is spec'd for, then you'll hopefully have normal flow and pressure and everything will be good from then-on.
But if the oil is still thicker even at running temp, then you've got less lubrication for the entire drive, under load, etc... obviously not cool!
Since oils are labelled at standard operating temperatures.
A 10W-40 for example, is a 40 at a standard temperature.
Your logic is that your oil is running slightly hotter than standard, so you'll run a slightly thicker oil (say a 50.), which will effectively be dropped back to a 40 (at your slightly hotter temperature.).
The problem is that most oils aren't labelled with a viscosity vs temp. chart, and that data can even be a real pain to get from the manufacturers.
So you really don't know (without a lot of research.) what happens to viscosity at the higher temperature you're ball-parking... the viscosity might stay higher than you want, or even drop sharply below.
There's shedloads of charts around from the old-school guys, showing temperature ranges and what-not.
They're either a pretty good chart as mentioned above for traditional mineral oils for back in the old days, or they're essentially showing which are the cheapest oils adequate for a climate.
ie, 15W-40 is adequate for a typical engine in Aus, and cheaper than a 0W-40, but not "better".
When you're talking ultra high-quality oils like a 0W-60; you have something that can circulate as quickly as possible, maintain a high viscosity at operating temperature, while also tending to be more stable at excessive temperatures.
But the oil system needs to be designed for such a high viscosity, you can't just whack it in an OEM system and expect an improvement.
Aanyway, so based on all this guff, I was convinced that a 0W-30 was perfect for hemis here. I figured 30 hot would get close enough to the spec'd 20.
But after trying 5W-20 thanks to guys like warod, my mind was blown
.
Much better economy (most likely just due to the MDS being happier.), much quieter and the hemi tick stops literally 2-3 seconds after start-up, even after sitting for weeks in winter.
I'll be going 0W-20 next and expect it to be better again (hopefully not tick at all on start-up.).
Sorry for the big rant everyone, but I've kept out of the oil debates for too long
. I don't expect anyone to change their minds, but at-least do some research (real research.) or experiments and find out for yourselves.
Aaand, at-least can we please stop this comparison of cold ratings.
ie, 5W-30 vs 10W-30... regardless of all of the crap above, there is no comparison - Lower cold viscosity is always better - End of story.
The only reason to use a 15W-40 over a 0W-40 is to save money, which is fair-enough, but don't be fooled into thinking that it's "better" for your engine.
I will check out the GK section at a later stage as you've suggested.
