Oil pressures on six cyl

My 66 convertible six cyl has about 70,000 miles. 10,000 on a recent valve job. I've been using synthetic oil since the engine was broken in with great results. I drive daily roundtrip about 40 miles to and from work. My question is; until a few months ago the stock oil pressure gauge showed the needle halfway across the gauge. Now it is towards the bottom part of the midrange. (There are two lines at the bottom, left side of the gauge and the needle shows just above the 2nd mark after the car is warmed up.) When the engine is cold the needle is halfway across the gauge. I live in Los Angeles. I've changed oil and filter. I've checked the oil sender connection. I often drive at 80mph, sometimes 85 (at 5AM) to work. Too much strain on the engine? I do this quite often. Could the oil pump be wearing out. Is it a life limited part? Other than that the car runs great.