Lost oil pressure in 2005 2500HD

you need to have engine checked and possibly rebuilt or replaced, this not a quick easy fix.

You talking crankcase oil? My info says 5w20 preferred. 5w30 can be used.
For the 2.7L engine, 4.7 quarts, that is with filter. Watch your dipstick when adding oil. Then start it, let oil circulate. Turn it off, give it time for the oil to drain back into pan and recheck the oil level on your dipstick. If you need to add a little to bring it up to full mark on dipstick, that's fine.

Oil pressure sensor, or most likely, oil level sensor, either sensors, wires, or connections.

The oil light comes on when the oil pressure sensor detects low oil pressure. The amount of oil has little to with it, although very low oil levels can cause loss of oil pressure. The best thing to do is to have the engine's oil pressure checked with a mechanical oil pressure gauge to ensure that the oil pressure is sufficient at all operating speed and temperatures. If the oil light is still flashing on and off and there is no indication of low oil pressure by the gauge, then replacement of the oil pressure sensor is recommended.

What is the mileage on the vehicle? What type of oil are you using, synthetic or conventional oil? Older engines that have a fair amount of wear (loose essentially). I had this problem on two different vehicles. If the oil is not near time to be changed (fairly clean oil) I would get a new oil filter and prepare it by filling it with STP oil treatment. This is thick stuff and will thicken your oil. Remove the old oil filter and put the new one on. Start the engine and let it idle for a minute then shut it off. Check the oil level and add more STP to bring it back up to the full mark. Start it and check the oil pressure. If this helps bring the pressure up then I'd say its time to go to a thicker oil. I switched to 0w40 synthetic after trying 5w30 (5w30 is recommended by mfr.) and have had no more trouble. The thing is that synthetic oil tends to thin more at temperature than conventional oil does.

could be the tappets/risers that noise is comeing from

I believe it's 4 quarts, but I always get 5 as an extra -- add 4 then start the car and let it run for a few seconds (~30 or so) then check level (while the car is level) and add as necessary -- a gauge of how much to add the amount between the low and high lines on the dipstick is 1qt - so if it's right at the low line, 1qt will put it at the high -- ideally, it should be in between the line.

yep you did!!!! why did you drive it with no oil presher!
oh well,,,,smack your legs!!!

If your oil pressure light comes on
Don't ignore it. An oil pressure warning light (or low gauge reading) means one of two things: either your engine has dangerously low oil pressure (for a variety of reasons which we'll get to in a minute), or the oil pressure sending unit that triggers the warning light (or operates your gauge) has failed.
The question here is whether you have a serious problem or a minor one.
First, do not keep driving if the oil warning light is on or your oil pressure gauge has dropped. Stop the engine, let it sit for a few minutes, then check the oil level.
Is the level low? If the oil level is down more than two quarts or no oil is showing on the dipstick, adding oil to bring the level back up to the full mark may be all that's necessary to make the light go out. Just keep your fingers crossed that you haven't damaged the engine from running it too low on oil. If you hear rapping or knocking noises the engine starts, you're too late. The damage is done and now you're stuck with the consequences.
Note: On some late model vehicles, the warning light will come on (or a message will appear) if a sensor in the oil pan detects a low oil level. The light is supposed to come on before the level gets low enough to cause any damage.
If your oil level was low, it means your engine is either leaking oil or burning it. Leaks can be fixed by finding and replacing leaky gaskets and seals. Sometimes the oil filter will leak if it isn't installed or tightened properly. But an oil burning problem means the valve guides, rings and/or cylinders are worn or damaged and more costly repairs are needed.
If the dipstick shows a full oil level, then low oil obviously isn't your problem. The oil pressure may be low because of a worn or broken oil pump, a plugged oil pickup screen in the engine's crankcase, possibly a plugged oil filter or excessive bearing wear. Or, the oil pressure may be fine but the oil pressure sending unit has failed. Further diagnosis will be required to determine what's wrong.
Should you attempt to drive your vehicle home or to a service facility?
It's risky. If the problem came on suddenly, your engine does not have a lot of miles on it (less than 60,000) and you noticed no unusual noises (no valvetrain clattering or engine knocking), there's a good chance that all that's wrong is the sending unit. But, there's no way to know for sure without actually checking the engine's oil pressure with a gauge, or replacing the sending unit to see if a new unit makes the light go out. If the problem is only the sending unit, your engine still has oil pressure and you can continue to drive it until the sending unit can be replaced. But, if you're wrong you risk ruining your engine.
If you think you might have a low oil pressure or oil delivery problem because of a bad oil pump or one of the other problems we mentioned, do not drive your vehicle. Doing so only increases the probability of further engine damage. Have the vehicle towed to a service facility for repairs.