Originally Posted By: ProfPS
Do filter bypass valves open when you start a cold engine? Not necessarily a winter start, but when started in the morning when the oil is at its thickest for the day. If so, since bypass' are set by psi and thicker oil increases psi, would a cold 5W-20 not open the bypass where a 5W-30 would at the same temperature?
It all depends on:
1) The oil viscosity.
2) The oil pump flow rate at start-up.
3) If the driver revs the engine up while the oil is cold.
4) The flow resistance of the filter's media, and how dirty/loaded it is.
The oil pump does not necessary need to be in relief mode in order to produce a large enough delta P across the filter media to cause the bypass valve to open. For instance, if the filter was loaded up and had lots of restriction, the the bypass would open with hardly any pump flow through it, and the pump not in relief mode. If conditions in 1) through 4) above add up in the right direction, the filter's bypass valve could open way before pump relief mode is achieved.