Our vans will typically either have an oil pressure gauge (most common) or a red oil pressure idiot light (like yours). The vans with idiot lights use an oil pressure sender that is basically a ground for the idiot light. When the oil pressure hits 4 psi or more, the sender begins floating the contacts and the ground goes away. That's why you get a red light when the key is in the run position (without the engine running). As soon as you start the engine, the ground to the light goes away and the light goes off.

My 89 van had the very same problem your van has (I would get a random low oil pressure light). So I installed an oil pressure gauge only to find the pressure was fine during these events. To further troubleshoot, I completely disconnected the sending unit wire and taped it off. With it like this I still got random oil pressure lights with about the same frequency. I suspect the wire between the sender and the light was chaffed/grounding, but the intermittent nature of the issue and the way the wire was hidden in the harness made it hard to trace. I remedied the problem by running a new wire between the gauge and the sender. That was about 6 or 7 years ago & everything has been fine since.

If you run with the bigger oil filter the van will hold approx 4 quarts, but what you're seeing is close enough. Your dipstick looks normal to me. Tim