The car is a 92 190E 2.3. I run 20W50 oil in the summer (70F-90F average temps here). Lately, I noticed it sometimes takes a whole lot more time to build oil pressure at startup. When I start the car with a cold engine but cold ambient temp, it will take about 5-10 seconds to register oil pressure on the gauge. It always did that and I think it is normal. The problem is when I come back from work at around 5 PM when the ambient air temp is close to 85F and the car has been sitting under the sun all day. Even if the engine is cold, it can take up to 45 seconds to register oil pressure on the gauge. I don't hear no abnormal sounds from the engine. Is this normal or should I be worried? Also, the car is dark grey (almost black), which I guess makes underhood temperatures a bit higher when the sun is out...

Extra info: Last November I was driving the car and noticed very low oil pressure at idle with a hot engine. Sometimes the gauge would drop to 0 (FSM says 0.3 is fine I think). This was with 10W30 oil because of colder temps (40F). I then parked the car for the winter and put a new oil sending unit in May. But I haven't got the chance to re-test the car in the same climatic conditions yet.

This engine only needs to last until this november, after that it gets thrown away.