Oil pressure isn't everything, its largely the last part of the process. Everything else determines oil pressure, its a passive value not an active value, thats why we measure it, because everything affects it.
Its been said over and over, but its important not to forget. You do not create pressure you create flow. Try to compress a spring lying on a table with one finger by pushing it sideways, it'll never happen it'll slide away. fluids are the same, just pressing on a fluid will only cause it to move, push harder and you can move more fluid faster. You can only create pressure by creating a restriction. Use your other hand to hold the spring while you push and now it compresses.
Applying that to oil, the thicker the oil the more resistance to movement the oil itself has, so the higher the resulting pressure, as largely the pump will try to move the same volume. The goal with oil is to make it as thin as possible while still being able to coat the parts. The thinner the oil the more can be moved through a given space in a set time frame, meaning higher cooling, better high rpm protection, faster debris removal, etc. Thats a huge part of why synthetic oils are so much better than conventional oils (additive packages aside as thats an hour long discussion on its own) owing to a term called shear factor/stability. The better the shear factor the better the oil handles under load (oil loading not engine loading).
When the engine was new, the factory weight oil created a set pressure, usually 60psi. If now using factory weight oil you're getting a lower pressure, the solution isn't to put a heavier weight oil in it. Since the pump volume capacity is the same (as long as it isn't failing) along with the weight of oil, the only other thing left is restriction to flow, that must have reduced. Which usually indicates wear, when you have excess wear the last thing you want to do is reduce flow by increasing oil weight as now that part has less oil to protect it. If you're pressure stinks, switching to a synthetic blend or full synthetic oil will help keep lubrication and cooling up while under low pressure situations, often times you can go one weight of oil higher without issue though. From 5W30 to 5W40 is common in older chevys that have horrible bearing clearance.
TL;DR Do Not put that 15W40 in it, thats way too thick when cold for the engine, you're starving it for lubrication when its most important. 5Wxx is the only oil you should be using in that new of an engine. 5W40 is fine if you want the extra thickness or if you're in a warmer climate, but 5W30 is fine for 90% of the applications out there.
