Low Oil Pressure - but only when warmed up?

[blindbug]

Does anyone have an idea on why engine oil pressure might drop considerably when the engine is warmed up? I am getting between 30-40 PSI at ignition and idle. However, when I drive down the road, it drops to 20-30 and once it gets warmed up (180-200F) I stop at lights and regularly get 9-10 PSI. Once I hit the gas to accelerate, the pressure jumps back up to the 20's.

The engine doesn't seem fatigued by the low pressure (it is from 1975, so it's a little worn out, but still running well). I do burn some oil, but not much, and there are some very slow leaks coming from god-knows-where. I can't tell that I have any exhaust leaks, or leaks at the header. Distributor is new (last 1500-2000 miles, so it should be engaging the oil pump without issue.

Is getting a low PSI like this in normal driving a common occurrence? Is it a sign of a larger problem that I might not need to address until its an actual problem? Anything I should check for or replace (maybe a different weighted oil?)?

A few details:

1. I have a new filter (K&N HP-3002) installed just over 1000 miles / 7 months ago.
2. I have had an oil change (Shell Rotella T5 10W-30) just over 1000 miles / 7 months ago.
3. I added an Oil Stop Leak (Lucas) when the oil was changed.
4. I have a brand new Dakota Digital solid state oil pressure sending unit hooked up to a brand new Dakota Digital Oil Pressure gauge.
5. I manually checked, and my oil Dip Stick reads right on full on oil.

Small Block 350 V8 from a 1975 C20, installed in a 1986 C10 Custom with a TH350 trans.