Oil + additive viscosity
IMO the one place where the restriction could be so bad as to cause the pressure to drop the way described is the spin on adapter MP used to sell. They were horrible design. So bad I think I threw mine out. The MP Engine book shows how to modify them for better flow, but it leaves hardly any threads left for the NPT fittings. The design was such that when the NPT fittings went in, the tips blocked the flow.
We can't see the OEM adapter in this photo, but from the spacing of the fittings, it looks like the adapter may have been a better piece than the aftermarket one. Canton and others now make a good adapter.
IMO the one place where the restriction could be so bad as to cause the pressure to drop the way described is the spin on adapter MP used to sell. They were horrible design. So bad I think I threw mine out. The MP Engine book shows how to modify them for better flow, but it leaves hardly any threads left for the NPT fittings. The design was such that when the NPT fittings went in, the tips blocked the flow.
We can't see the OEM adapter in this photo, but from the spacing of the fittings, it looks like the adapter may have been a better piece than the aftermarket one. Canton and others now make a good adapter.