Oil pressure/gauge problem
 

I've got a 2002 Express 3500 V8, 5.7 liter, about 115,000 miles. Would love any help with this!

1. I bought this used, and it's always been at 40psi when accelerating, and then drops like a rock to 10-20 when idle (not slowly, the needle just drops).

Is that normal?

2. A few days ago, it started dropping to zero at idle, and then 10-20 when accelerating. I checked the oil and there was barely any in there. So clearly there's a leak somewhere. I refilled with oil.

3. Now, it starts at 40 and stays there until the van warms up. Once it warms up, it stays around 40 when accelerating, and then drops to 10-20 when idle.

Based on that info, any idea WHAT exactly might be leaking? Do you think it's a combination of a leak and a sensor? The engine sounds perfectly fine, even when it dropped to zero.

I have the same van....almost. Mine did the same thing. When running the pressure is fine, but at idle in drive at a red light it would sit at the top of the redline at the low end of the gauge.
I fixed this by switching to high mileage oil. I think the combination of the wear on the engine, and the oil pump, that the thinner regular oil is not quite enough. Not to mention that they are reducing the ZDDP in oil nowadays to be more compatible with the newer emission systems on cars......so lets have our engines pollute a bit less, and wear out twice as fast......bs.
Anyway, high mileage oil has more ZDDP with is an antiwear additive. (zinc phosphate) and since I switched, my gauge reads higher at all times, idle or cruising, and there is more left on the dipstick when I check it at the oil change time.
I will never go back to regular oil.....plus, I read that you can't since the extra ZDDP also swells seals and helps to eliminate leaks, but then switching back those seals would leak more that they did before.

Thanks, I do actually put high mileage oil in. I guess I'll just have to do this via process of elimination. First step, mechanical oil pressure gauge.