Oil Pressure on my 16v engine

Just a quick one really. I've installed a oil pressure gauge in my car, and have put the sender unit right by the oil filter. Basically it starts off with good pressure at around 70-75 psi, but as soon as the engine starts getting warm, the oil pressure seems to drop to around 15-20 psi on idle. Is this normal or is something wrong? It's a fully rebuilt 16v kr engine on zx9r bike carbs.
It's running 15/40 non synthetic oil, so I've eliminated it being a poope engine or bad oil. Help is needed

may need this moving to the carb section although it was originally a fuel injected engine