Oil Pressure Gauge reading dropped

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I have a 1990 300TE4MATIC wagon with 125000m, oil changes every 3000m or 3 months. My oil pressure gauge has always read normally, 3.0 bar under load and below 0.5 when idling really hot. It has been hot here (mid 90's). Driving over home over Teton Pass 95 degree day, engine temp at 100+ F, auxiliary fans on at one point, 10% grade, I noticed that the gauge under load (4000-5000 rpm) dropped slightly off 3.0. Now when I start it cold, it no longer pegs to 3.0 and runs just below. Oil levels are normal; engine sounds fine and otherwise appears to be running normally. I'm due for a change, so will do that first, thinking it could be bad oil filter? I'm concerned about the valves, possible engine damage or could it be an oil pump issue? Need advice. Thanks in advance for any input.