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One of the biggest differences between a standard Fram and the HP series fram is the relief valve pressure and micron rating.
Most standard Frams (PH5, PH30 , etc) have a 20-25 micron rating and as stated in prevoius posts don't have the best glueing techniques and thus the cardboard joints can fail.
The HP series have a 18 micron rating and also has a filter relief valve in that if the filter media becomes plugged, then the relief valve opens and the oil still gets filtered through an steel screen.
You will pay about double for an HP4 over a PH5 but it is well worth the money. It is a different breed of filter than the standard Frams and not a really good compariosn. I have been using Fram HP series for 15 plus years and have yet to have an oil related failure in 50-60 BBC engines.
Just my .02
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