I installed an oil cooler in my truck, the type that sandwiches off the filter mount, and ran lines to the little cooler. My top end oil pressure dropped by 10 psi, but my pressure at idle stayed the same.

Can anyone explain why?

 

If you have a mechanical gauge,the type with it's own oil line,the length of that line can affect oil pressure at the gauge.Maybe the lines that you added for the cooler are somehow doing the same thing,or maybe the cooler is getting air locked and affecting the pressure.It may just be affecting gauge read out and the actual pressure to the bearings is still the same.I have put in several oil pressure gauges over the years and I have heard people say you have to bleed the air out of the line,but no matter how well I bled them out, they always ended up getting air bubbles in them sooner or later.You may also want to keep in mind that you can run your oil too cool,which can cause a lot of sludge build up.Engines used for short distance runs all the time,like town driving,have a tendency to do this.The oil never gets hot enough for the detergents to remove the sludge build up.It takes a good 10+ miles to bring oil up to engine operating temperature.Something that a lot of people don't know,is that if you run an engine with no thermostat,it can overheat because the coolant dosen't stay in the radiator long enough to cool down.That has nothing to do with this,but it is interesting.Just for the heck of it,feel your cooler after a little trip and see if it's hot, if not, it may be air locked.