[Spridgets] oil pressure question

[uniquelittlecars at yahoo.com]

My car has been loosing oil pressure if i drive at 70mph very long.
At first i thought it may be the oil pressure release valve.
But it seems to be working at lower speeds.
It starts at 60lbs at start when cold.
Then runs at 40lbs as long as i do not run over 50mph.
What is a acceptable drop in oil pressure.
Mine at 70 mph drops to just over 20 lbs.
I am wondering what others have for oil pressure after rinning at these
speeds.
I am not running an oil cooler on the motor.
My water temp is close to the N for normal on the gauge.
The lower end was done in the last 1000 miles. New bearings.
The crank was not warn and the bearing were in need of replacing but not
gone.
When the motor was put together the crank was not turned just new bearings.
After the car cools down it runs at 40lbs again until i go high speed again.
Is this common with a 1275cc motor?
If not - any guesses on what is going on with the oil pressure.  
 
Vigil