Oil pressure lamp is on. Looking for a reason

Hello. I have a VAZ 2115, injector, 8 class, 2002 onwards, 204000 km mileage. The engine feels already worn out. Help me to understand. I had the following situation: after 8000 km of run I buy the same oil (zik 10w-40) and a filter.

After changing the oil, everything was fine. I drove around the city for about 2 weeks (I drive about 20-30 km every day) and when the engine was started in the morning, the oil pressure lamp started to stay on for about 3 seconds.

Then every day it takes longer and longer. As a result, I came to the conclusion that during the morning start it burned for about 12 minutes. Then I drove along the highway at a distance of 100 km. On the way, the lamp started blinking and eventually came on. I turn off the engine, wait a couple of minutes, then turn it on and the lamp goes out. Then, after a while, it flashes again and lights up.
First I tried to replace the oil filter. Did not help.

Then I tried to replace the oil pressure sensor. They took it off a friend from his working car, put this sensor on my car and everything is the same: The lamp burns when the engine is started, then after a while it goes out. Then he removed the valve cover and rinsed it well. Then he took off the multi-layer mesh from it, soaked it in kerosene, washed it well, cleaned it and put it back. It seems to have helped quite a bit, but the problem persists.

Then I went to a familiar garage mechanic and we screwed in a pressure gauge instead of an oil pressure gauge. Cold engine pressure 3,5; hot 2,4. He said that this is the norm. But the problem remained. It didn’t seem to be hitting the pallet, so it should be intact, and besides, there was protection. Now I'm going to remove the oil pan and watch the degree of contamination. And also wash the sump and oil intake. Maybe someone has encountered such a problem? Tell me what to do?
By the way, I've been driving this way for weeks 3. so far, the engine has not knocked)))