Very low oil pressure! Tapping!

At first I was using 10w-40 oil, and my friend told me that I should use 20w-50, and I don't really know anything about oil weights, so I listened to him. After changing the oil twice, using the 20w-50 Castrol, I switched back to 10w-40 valvoline, and my oil pressure dropped dramatically. My lifters are tapping really loud, and my oil pressure drops to about 5lbs. of pressure when hot, at an idle, causing the check guages light to come on. Should I replace the oil pump? Or what else could cause this drop? Please help.

What year and how many miles are on your truck? From the sounds of things you may need to overhaul your engine.

How was the oil pressure before you switched to 20-50?

At first I was using 10w-40 oil, and my friend told me that I should use 20w-50, and I don't really know anything about oil weights, so I listened to him. After changing the oil twice, using the 20w-50 Castrol, I switched back to 10w-40 valvoline, and my oil pressure dropped dramatically. My lifters are tapping really loud, and my oil pressure drops to about 5lbs. of pressure when hot, at an idle, causing the check guages light to come on. Should I replace the oil pump? Or what else could cause this drop? Please help.

First off, don't listen to your friend anymore, he really doesn't know how he is talking about.

Use 5w30 in the engine, nothing more, nothing less.

What oil filter are you using?