8.1 Oil Pressure and Filter

Looking at where the filter is located, and where the sender is located it is most certainly post-filter. There is not one single reason to internally flow unfiltered oil up to the valley and back down to the filter, just to measure pressure pre-filter. Pre-filter and bypass pressure is really useless anyhow, it can potentially be very misleading to actual oil pressure, since a plugging filter will show high pressure and decrease pressure to the engine.

This is a SBC diagram, but how the 8.1L probably is plumbed as well. You can see the oil pressure sending unit in the top right of the diagram, post-filter.