on cold start(25)low oil presure warning for about 2 seconds untill guage gets to 20psi

Its possible your oil pump is showing signs of age. I am assuming this is the 6.0L gas engine and not the duramax based on the amount of pressure at normal temp. How many miles on engine?

yes 6.0l 119,000 miles. Had oil change last winter, shop used bulk oil, problem started next day. Changed back to gtx prob. stopped until this winter.

Wow. That seems very strange but I still suspect the pump. Evidence of it with the problem starting again in colder months with the oil getting thicker and taking longer to travel.

changed oil ( after only 800 miles ) to syth, blend,5-30. problem stopped, got quick presure on start to 40 psi but ran high on highway( 50+@2500rpm) after one week got slow presure again on cold morning, (10-20f)