I have a 2000 blazer lt 4wd. when i start it the oil pressure reads 40. after i start driving it slowly drops to 20 asnd fluxuates around there while i am driving. once i stop at a red light it drops to almost 0 but goes back up to 20 when im driving. when i stop and put it in part it drops to zero again and when in part i rev the engine it goes up to around 20. what could be the cause of this. and oil is at proper level.

Dear Bill, You will need to find a mechanic who can remove the oil pressure sending unit from the engine block and insert a "real" oil pressure guage to tell if you really have a problem or not. Lots of factory dashboard guages are not very accurate over time. If you really do have that low of oil pressure, you need to address that quickly.

do you change oil at recommended intervals when the engine runs at higher RPM's it's normal for oil pressure to increase while driving

Oil pressure always changes with rpm.
At idle you should have around 10 psi, and it should max out at high rpm around 50 psi or so.
You not only can't have as much pressure at idle, but you don't need it either.

4.3 V6?
With the engine hot, between 6 to 40.
6 during idle, at a stand still. 40 once its at around 4k rpm's.

I have a 99 blazer and ever since I have owned it, it reads lower in pressure when idling, but will go up too 40 while driving. This is normal for the vehicle. If the pressure gets real low, and stays low, it means its time for an oil change.

As long as the pressure goes up when you give it gas, it is fine.

There may be a loose wire or the gauge or sending unit may be going bad. The first thing I would do is to hook up a temporary oil pressure gauge, not using the old one at all. With this watch gauge while driving and see what readings you get. If the readings still jump around you have a pump that is getting weak and failing, If that is the case the pump should be replaced, but with high milage you may consider rebuilding the entire engine. If the readings stay normal start checking for a loose wire at gauge or sender. Then check sender, then gauge and replace as needed.

your oil pump does that on purpose - more noticeable with a dented oilpan as pressure is slightly increased. nothing to do until it goes (if it goes) unless you want to throw parts at your car anyways.

The oil pump is in the oil pan and needs to be dropped and being a 4WD you also need to drop the front axle pumpkin to remove the oil pan and replace all the gasket needed. Start with the easy and replace the oil pressure sending unit, it's on the back of the engine to the passenger side of the distributor. You can go to Auto Zone and pick up a new Oil sending unit, just make sure your get the right one, "Sending Unit" Looks the same are the oil pressure sensor but the unit reads the pressure, the other just signal a light to go on and off. It's an easy fix like replacing a spark plug. 

I have a gmc envoy (same thing) and it does the same thing. normal.

If grounding the dash does not work, try putting a mechanical gauge on it and reading actual oil pressure. Pressure does change while driving, but should not wander. With mechanical gauge, check that running hot at idle (in gear if automatic) pressure should not drop lower than 15psi (I'd rather see 20) High reading should be around 40 -50 but no higher.Low pressure indicates a loss of supply or bad bearing on the crankshaft, high pressure may be caused by sticking pressure relief or restriction in an oil passage.