Can anyone explain in laymans terms what the relationship is between the viscosity of oil and how that effects oil pressure or is there no relationship between the two, I just need to know 









                                  Mmick (I wonder how many will look at this and shrug their shoulders)

It's a bit like Diarrhea and constipation the thinner the shit the easier it flows 

Lets say you use a 10W 40 oil, when you pour it from the bottle it's reasonably thick, but when you drain it from the bike hot it's very thin and runs easily, the viscosity rating is a sliding scale, relying on the temperature of the oil as a key factor in measuring it's viscosity, the wider the range between the two numbers the more stable the oil is. i can't remember what the exact temperatures are for the measurement but for arguments sake lets say it's 0 deg C AND 100 deg C., at 0 the oil will have a viscosity of 10,the W is a winter rating and indicates it will pour or circulate at a set minimum temp relative to the claimed rating of the oil, SG, SH etc. At 100 deg C the same oil will have a viscosity of 40. remember though, these ratings are relative to the temp so to get the same thickness of oil at 100 deg C as at 0 it would have a very high viscosity number.
   Obviously, the oil pump has a static setting for the gears, the oil pressure rises and falls in unison with how fast the oil pump is turning, the variable is the oil and how it behaves at temperature, if you use a 15 - 60 the variation in actual flow rate of the oil will be less than if you were using a 20 - 40.
  Oil will lose it's effectiveness through usage mainly because there are plenty of additives used to establish critical measuring criteria that are needed to pass API standards and therefore gain the minimum std for the time, again, SG, SH etc. additives like detergents to help keep the engine internals clean, dispersants, to encapsulate foreign particles like carbon etc as well as long chain polymers that are a cheap way to help establish a viscosity rating. All wear out, all have a finite lifespan in an engine and they all contribute towards how an oil performs, The higher the quality of the base oil, the fewer long chain polymers are required to make a stable product, the better the base oil (mineral or synthetic) the more shear stable it will be as well, as for the other additives, it depends on the application as to how much is added, in a diesel engine for example, with high compression blow by into the crank cases is an issue so very high detergent/dispersant packages are added, worth noting here too, when you drain your oil if it's relatively clean it's not done avery good job in keeping the inside of your engine clean, even if it's brand new, as you use it you're contaminating it with by products so the usual rule of thumb is the darker the oil you remove, the better job it's done keeping your engine clean.
 
paul.
look here for the graphs which help demonstrate what i'm trying (poorly) to explain. it offers a better explanation than mine too.

http://www.upmpg.com/tech_articles/motoroil_viscosity/