@Mark and Driven slow:
The bypass valve opens when the pressure difference before and after the filter material is too high. Typically at a pressure difference around 1 Bar or slightly higher. A filter in a standard situation with regular oil changes will become clogged beyond 100000miles or so. Clogging is not any concern at all if the filter is regularly changed with the oil.
The bypass valve has an important second function which is often overlooked! When a cold engine starts with the oil drained back and air in the filter, the incoming cold oil has to pass through the filter paper. This presents a major resistance. Thus it takes some time before the oil channels are filled with pressurised oil. The starting engine exhibits a " cold start no oil rattle". With a bypass valve the pressure on the filter paper is relieved during cold start and that way the oil channels achieve a normal pressurised oil condition much earlier. In short, the bypass valve shortens the cold start rattle period by providing proper lubrication sooner. That's the chief function of a bypass valve.
The anti-drainback valve is actually a simple cover over the intake holes of the oil filter. There is a plethora of material and shapes. The best and most expensive material is PTFE, as in radial shaft seals. However, many filter makers simply use cheap rubber. Rubber becomes hard in the cold, deforms after being heated, and has much less flexibility. Rubber is no option, it is highly inferior for that purpose.
The best oil filters include all three features. Those being a PTFE anti drainback valve, a bypass valve, and the finest filter material as well. Since a finer filter material automatically has a higher flow-resistance, such filtersmust have the most efficient bypass and anti-drainback valves too. Just one of those features is no good and not enough, all have to be up to par, thats what making a good filter is all about.
I agree that the filter material, anti-drainback valve, & bypass valve are the three most important parts of an oil filter. Most manufacturers emphasize the importance of the anti-drainback valve for cold starts.
I'm not sure how much 0W-20 oil is blowing past the bypass valve during cold starts, unless temperatures are really low?