Oil pressure

The oil pressure lamp comes and goes when the oil is cold, even in high RPM. But when the oil is warm, it seems to have enough pressure. During the summer period, there is less problem, but now when it's 7-8 deeg outside (starting oil temp 15 deeg) it doesn't feel right. I use SAE 50 oil.
It use to be other the way around....Does anyone have an idea?

First thing that comes to my mind is to temporarily add a mechanical gauge to verify what you see via the warning light.