The bypass valve responds not to your oil pressure, but to the pressure accross the oil filter. If the oil filter gets dirty, the pressure across it increases meaning pressure drop to the pressure lubrication in the engine. If your oil pump produces 100 PSI and you have a pressure drop of 30 PSI across your filter, you only get 70 PSI at the lubrication points. A bypass valve would mean that some of the oil going through the filter will be unfiltered NOT ALL, if all oil was unfiltered, there would be no pressure accross the filter to operate the bypass valve.
In any case, on an old engine that is worn a filter with a low bypass valve pressure would be preferable, since it would mean more oil pressure was available for lubrication (oil pressure in an engine tends to fall with age).
As for Purflux filters, they are the OEM filter for my car, so in buying Purflux I know I get the right filter since it is the one fitted at the factory: it costs no more than the equivilent pattern filter.
If you know the part number of the original filter for your car, when you ask for a new filter, you want one that is explicitly shown as being equivilent to it, i.e. if it doesn't say in the filter catalogue that the filter you want is equiv to the oem filter, don't buy it.
Purflux filters
Moderator: RichardW