A full flow filter on a stovebolt engine is a solution looking for a problem. It sounds like your oil is a bit too thick for the climate, or your filter media is woven a little too tight. At least you also added a bypass filter to clean up the oil efficiently. I'd suggest one oil change using 10W-30 oil to see if the pressure comes up quicker. If so, you need to run thinner oil in cold weather. If you don't want to deal with the hassle of changing the Frantz filter element, spin on filters down to the 1 micron range are available to use in the bypass system. They're normally used for hydraulic fluid or Diesel fuel, etc. DO NOT use a tight weave filter in a full flow application.
Jerry

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