Well first thing, once you go synthetic your not supposed to go back to normal oil. Synthetic has detergants which clean out the engine and help protect against friction better than regular oil does, it also conditions seals but if the seal is allready shot it cant do anything for it. On an engine with 100,000 + miles thats not a good thing. The synthetic will break down any deposits and gunk inside there, and they will all float around in your crankcase until the next change. You dont want tar and chunks of burnt oil floating around in your engine. It will do this until the engine is clean, but it will also eat away the gunk around the seals, which maybe holding back a huge leak.
Why would you use the same weight synthetic in the winter as the regular oil in summer? That doesnt help it any. Your supposed to use higher weight oil in the summer.
For the oil pressure problem. You may have to much oil in it, or your oil pressure sensor is fudged. Most likely the sensor, but check the oil anyway.