FWIW, not all aftermarket filters are created equal. Many of them do have anti-drainback diaphragms, but several do not. Napa Gold and Wix (actually the same filter) are high quality and also have this feature. Some of the cheaper filters may or may not. You can tell by looking through the small holes of the filter. If you can see up into the filter, then it doesn't have the anti-drain back valve. If it looks like these holes are blocked by a piece of rubber, then it's got it. There can be other issues with cheap oil filters (like open areas between pleats). I like Toyota because I never need to wonder. You might be pleasantly surprised by the cost of Toyota's filters. For some time now they have been selling these for only $3 - $4 USD.

Not having an anti-drain back valve (diaphragm) would not effect pressure except initially when you 1st start your engine. This is because the pump must fill the filter before it begins to circulate in the engine.

Personally I think 20w50 is too thick for the van. I might make an exception in a warm or hot climate IF the engine was worn and couldn't maintain enough pressure otherwise. If you have any doubts about oil pressure, rig up an oil pressure gauge and take the guess work out of it.

Having too thick of oil can actually starve your engine for oil. If this oil is too thick you may not get enough volume through to the bearings and other small passages. Another potential problem is the pressure relief valve on the oil pump. If pressure gets too high, the relief opens and spills the excess oil back into the pan (instead the pump pushing it to the places it's needed). Tim