2004 Voyager RG oil problem and door lock problem

Hi, I have a 2004 RG Voyager. I have two problems that I can't find a solution for. The first is that I just had an oil change, 10W-40 oil was put in, and it has 340,000 km on it. Occasionally, the oil pressure light flashes, and the level is about 3/4 below the maximum. Typically, the light comes on when the engine is warm (after 5-10 km) and I have to stop at a red light, for example. The RPM drops below 1000, to idle speed. If I gently step on the gas and rev it up to 1100, the light goes out. This did not happen with the old oil (5W-40). Could this mean something?
Another thing is that after I lock the doors with the remote control, the horn and alarm go off within 2-5 minutes, at which point I open and close them and they don't happen again. What could be behind this? Could you give me some advice? Thank you in advance.