Sudden Low Oil Pressure - TRactor Engine

[John & Patricia Donnelly <pdonnel1@san.rr.com>]

HI Everyone,

After a 2 hour drive at freeway speed the overall oil pressure has dropped 
significantly. Before, the oil pressure ran at 70 PSI at 3K+ rpm, now it 
runs at  20 PSI. The highest it'll go is 40 PSI at 4K rpm. Not good.

Here's my guess, but I'd like some opinions before I start replacing parts. 
I think that the screen on the oil pump has clogged, so the normal oil 
pressure can't develop.

Any other ideas for the sudden drop off of pressure?

Thanks!
John
'67 TR4A IRS
Pretty much original, but has spin on oil filter