Since the oil filter takes a tiny volume of oil from the main oil gallery, strains it a little, and dumps it back into the oil pan, it has no effect at all on oil pressure. A lot of stovebolt engines were sold without a filter, since it was an extra cost dealer-installed option. The most common cause of low oil pressure and/or slow pressure buildup is worn connecting rod bearings. Drop the oil pan and inspect the bearings before you do any more damage to the engine.
Jerry
