Vacuum sensor on oil return line

Sorry Peter - no link from me but I am sure the information is out there - its the relationship of viscosity and temperature. Oils viscosity goes down as the temperature rises (it gets thinner). As it thins the ability of the pump to maintain the cold(er) pressure drops. This is why cold engine start up usually sees a relatively high oil pressure that then reduces as the engine/oil  temperature rises.

So, if you see an oil indicator pressure drop but no rise in oil temperature you most likely have a faulty pressure sensing system. The corollary is also fairly true.