So in comparing a lot of aftermarket oil filters to OEM oil filters for my Subaru and Ford I got to wondering (again) about the relief valve opening pressure. Do OEM filters have a range like aftermarket filters? Or, do aftermarket filters have a range of PSI because they aren't built for just one specific car?

For instance, for my Ford F150 the applicable PureOne (PL24651) has a relief valve PSI of 8-16 PSI. Does that mean that the valve starts opening at 8 PSI and is fully opened at 16 PSI?

Yep.

i so smarts

Keep in mind that is the pressure differential across the filter element (into the filter vs out of the filter) and you're unlikely to hit even the lower number under any normal operating conditions.

yep

So, assuming that the oil is at op temp, and the filter is relatively new, it won't go into bypass even at WOT?

It wouldn't matter if the oil was one degree above the pour point. The largest restriction against the flow of the oil is the passages in the engine. By comparison, the filter is much easier to get through. If the oil going into the filter is an amazingly high 100psi, it just has to leave the filter at at least 92psi to stay out of bypass.

The bypass is a hail mary effort to protect against the possibility of clogged elements, not for easier flow at startup.

People have a hard time seeing the direct kinetic relationship that oil has on either side of the media. The stuff plays bumper cars throughout the entire engine. If it can't fit through the engine, it's going to back up (in pressure) to the pump. When the pump breaches the relief setting ......THAT'S when that 1:1 kinetic relationship is severed and stuff doesn't HAVE to go down the pipeline.

The filter is nothing to flow in itself. For all the pressure you apply to the upside of it, the back side has just as much (near enough) pressure to "shore it up". Back pressure. It's only in relief (or when the gallery is empty) that you can alter this extremely close in:out pressure relationship.

Naturally ..this is outside of "unicorns".