The oil pressure gauge always shows maximum (right side position), when the engine is running and shows 0 (zero), when the engine is not running?

Hello, I am writing from Vilnius (Lithuania). I have a problem in my car. I changed the sending unit, but the gauge shows the same. Then I verified oil pressure with another mechanical gauge. The oil pressure is about 24 psi at idle, about 35 psi at 2400 rpm and about 40 psi at 3400 rpm, when the engine is warm. I think that oil pressure is normal. What happened with oil pressure gauge? Thank you.