In theory, the thinner the oil is, the less turbulence you’re going to create in the oil channels and valleys. This will allow you to get oil, to where it’s needed, faster. Where as a thicker oil, in theory, will via friction, “bind” the system up more. Attached is the math to do a simple Reynolds’s number. However, an engine is a complex system - not just a linear system. So the math isn’t straight forward. You would have to do it for each part of the engine. (Which, I assume the engineers take this into account. I could be wrong.)
Given an engine is a dynamic sump, you’re not going to have instant film thickness everywhere - as we don’t have a pre-pump to develop a film on the bearings. As well, part of the engine is lubricated by gravity of the oil.
www.stle.org
As we can see from this article, the majority of wear still comes from lack of lubrication. For the first few critical seconds, you have lack of lubrication. The faster - even if it’s fractions of a second - you can get proper boundary lubrication to critical parts, the better off your engine is.
Also, while not straight forward. Filtration effects flow rates.
Question: Joel White of Weyerhaeuser was curious about the effect of flow rate, contaminant level, and viscosity on the performance of a filter, comparing the multi-pass Beta test (ISO 4572)...
www.machinerylubrication.com
It does not effect filter performance unless you’re in a condition of extremes. But filters, will slow flow as well.