I ask this since adding 2 quarts to an engine the oil pressure reading has spiked.
Then again some here code Mobil 15W40 more akin to a 10W40 to its low temp pumpability and 100C Temp cSt rating.
What is the verdict?
which bypass, the oil pump, or the filter?
there's a relationship between viscosity, pressure, and flow. as far as physics goes, it's a relatively simple relationship, so we can make general statements.
basically, the bypass is to pressure differential, and unless you're overrunning the bypass flow capability, you aren't changing the bypass pressure setting.
what you are changing is how much flow there is before the system goes into bypass.
if it's stone cold, i think it's possible a really thick oil might overrun the bypass valve in the oil pump. so if the spring is set to a 60psi bypass, you might actually see more than that when cold.