oil pressure light on-question

[downbytheriver]

68 c10, 250 six

I fired my truck up today after it sat for the past two days. The last time I drove it, it gave me no problems. Today, as soon as I turned it on, the oil pressure light came on. I turned it off and checked the oil, which was full. No leaks or puddles. I then drove through the neighborhood and noticed it was running a little rough, so I took it home. If I revved the motor, the light would dim in relation to the RPM. It has rained a lot in the last two days, and the truck has been outside if that makes any difference.

My thought is to check the pressure with a mechanical gauge tomorrow, and replace the oil pressure sending unit, but I wanted to see if anyone had any ideas on other things to check. Thanks fellas.