What might cause the drop in oil pressure?

I have just gone through an engine overhaul of my 1994 Toyota Hilux Surf after realising loss of power which was attributed to worn out piston rings. However, now the engine oil pressure which reads within the normal range at idle, drops when the engine gets hotter. Actually dropping to near zero.
The turbo is also known to be worn out, but we are still pooling resources to fix that. What might cause the drop in oil pressure?

It is not unusual to have low engine oil pressure when you start a cold engine. Ordinarily, within 15-20 minutes, a normal engine should have built sufficient oil pressure as it warms up.
This should be confirmed by the oil pressure gauge resting at the midpoint. In the event that the oil pressure drops as the engine warms up, you need to pay attention to the oil pump and pressure relief valve.
The oil pressure pump plays a crucial role in the regulating of engine oil pressure.
If engine oil pressure pump fails, it will not be able to build sufficient oil pressure. Your mechanic will also need to check the oil pressure relief valve.
This valve regulates engine oil pressure depending on the flow rate. Engine oil filters have oil pressure relief valves which will allow engine oil to continue blowing in unique circumstances such as when the oil filter is clogged with aged oil.
This can lead to oil pressure dropping as the engine warms up.
Other causes of low engine oil pressure are worn-out crankshaft and bearings.