TallPaul is of course right. But I'll add something - perhaps not asked, but relevant.
Thicker (higher weight) oil results in higher oil pressure due to it's inherently higher resistance to flow - thats what causes the higher oil pressure in the first place. Reduced oil flow does not automatically mean better lubrication. Thats why thick oil on cold mornings can cause high engine wear during startup - because the resistance to flow causes the oil to not reach moving parts quickly. The proper viscosity is a balance to achieve proper flow at the correct pressure over different temperatures to adequately lubricate all moving parts. Thats why manufacturers have different specs for the oil to use in each engine design.
Agreed. Thick oil will give you higher pressure, but a lower volume of flow.
The lower volume is bad for cold starts. I've also heard that a lower volume is less effective for cooling, which makes sense, but I've never seen any scientific proof of this problem.
I'll agree that more flow should give better cooling.
But doesn't the thicker oils resistence to flow mean its trapped in the bearings giving a thicker cushion of oil?
I imagine it like a bike with a leaky tire for that for some reason is filled with oil. This bike has an oil resevoir and pump that can fill the tire as you ride. The thicker the oil, the slower the tire leaks giving higher tire pressure. Thinner oil leaks out faster giving the tire lower pressure. Either oil in the tire is fine for riding down the street until you hit a sharp edged pothole, which oil do you want to have in the tire to avoid denting the rim? Thicker higher pressure oil, or thinner lower pressure oil? I'd rather have more pressure in my tire just in case...
This little analogy ignores hydraulic locking effects of forcing oil through a little hole quickly but for me it shows the advantage of thicker oil in a motor. What if your engine pings for some reason? What do you want in the rod and crank bearings, 5W20 or 15W40?
I'll pass on another thick vs thin thread, and stick to what the mfg recommends for climate and conditions I drive in. I don't know enough about hydraulic locking effects to comment about them. But like with some many other things on this board I'm eager to learn.
There's very rarely a set of elements that are all things in all seasons.
You can spec an oil that cannot be fit into an engine at startup visc at all temps.
Said another way, you can spec an oil pump that will be in relief at initial start due to the viscosity requirements of warmer operations.
Sometimes you design an engine with volume and visc requirements that can't be totally managed at start up temps at the present levels of VI for lubes.
Pump volume, pump relief, and the engine's capacity for throughput don't have to have any rational relationship to each other ..at least on the high side. The relief limit may be due to stress limits on the oil pump drive. I've operated more than a few pieces of equipment that had a normal operating pressure that never dropped from cold start pressure ..yet they may require high viscosity oil that will never allow the pump to ever be out of some state of variable relief.
I'd say that one needs to qualify many things as to "when" you're talking about flow and pressure. They can't always fall in all the circles of a Ven diagram ..or rather all three (or more) circles can't sit on top of each other at all times.
The thicker oil COULD be a better cushion on the bearings, but it has to get there first to be of benefit.
But plain bearings never touch [at least they shouldn't!]. The mechanism relies on not only the relatively puny oil pressure, but the hydrodynamic wedge that the journal/bearing create under load. It is like a mini oil pump.