go to motor oil guide.com and you should find the SAE about oil. Yes always change thh filter when changing the oil. drain the oil first, leave the drain plug out then change the filter in the canaster. You should get some additional oil out. Install a new filter then fill with oil. Note if the oil pressure is dropping when it gets warm to hot I would say you have worn internial engine parts/ bearings, oil pump? Going to a non detergent oil may bring up the pressure???. Nothing will provide a good result if parts are worn internially. I straight oil has less vescosity then a multi oil. Going to a heavier oil will bring up the pressure some but then again heavier oil takes more horse power to move it. As for going to a detergent oil in a non detergent oil/ rule is that the detergent oil will remove the oil glazes off the bearings and what not and create excesive clearance in the bearings causing the oil pressure to drop. Plus creating more bad things to circulate in the oil system causing more excesive wear on parts.This is my thoughts to keep it short so hope it helps. Good luck.