Not on the stock oil relief valve.

There's a spring and plunger under that cap.

All you can do is check to make sure that the plunger isn't stuck/seized.

I did see on a post over at the pirate4x4 forum about an adjustable relief cap that allows you to raise the oil pressure, I don't know if it will let you lower it though.

Chucky MoMo <img src="/forums/images/graemlins/wink.gif" alt="" />