Low oil pressure when warm

Hello,
I'm a farmer and have a sprayer with a 300tdi engine in it.
When its started in the morning everything seems normal then when its warmed up a bit the oil pressure light comes on at tick over. When the revs are increased to just over 1000 the light goes out and everything seems fine.
I've had a pressure switch but nothing changed.
Thanks

Change the crankshaft bearings and oil pump. Possibly use a thicker oil.

Sounds like it is worn.

Have ye a nosey through this fred chap.. http://www.landyzone.co.uk/lz/f8/200tdi-oil-light-flickering-what-next-check-191739.html