Originally Posted By: FoxS
I was thinking that thinner oil would flow faster and transfer more heat into itself thus delaying how fast the engine got to full operating temperature.
Even if it flows faster the engine itself only has a certain amount of heat to offer and the oil remains largely within the confines of the engine block, which takes considerable time to heat itself, plus all the coolant. Also if you have any sort of oil cooler, fast oil flow will make heating take longer since it will flow faster though it and into the air, which will dissipate away from the engine.
Originally Posted By: INDYMAC
I would expect the higher viscosity oil to heat up faster due to its greater resistance to flow, thus creating more friction. But the 40 will never get as thin as the 20. So if fast oil temp is what you want, I suspect the 40 would be the winner.
This is the only real practical explanation for different oil causing any measurable sort of "heatup" difference. The oil being heavier makes the engine work harder, the engine working harder to pump oil will heat it up, and battering thick oil by forcing it through an engine will cause it to heat by the property of "working" the oil itself mechanically. The difference this causes though will be absolutely minimal, if you can even measure it. Plus, it will take longer to heat up to a level of fluidity where its not robbing the engine of power considerably. It stays thick longer, so even if it heats quicker, you've lost any benefit of the fast heating.
Besides, none of this accounts for multigrade oils, which completely throw variables into the mix, since they all start off at considerably thinner (0W- 5W-) viscosities, so the heating from mechanical working of the oil is gone.
A 5W-30 will need less heat to match a viscosity a 5W-40 needs, but both will, essentially, follow a fairly consistent heating curve when they're cold.
Thinner oils hit a point, faster, where they're no longer robbing as much engine power as a thicker oil would. That's the only difference. An engine puts out only a certain amount of heat and you're warm up times are fairly limited by them.