low oil pressure
 

i've got a 98 stx reg cab v-6 135k...

once the engine is fully warmed up the oil pressure drops at idle(light comes on), then the idle gets rough. the oil pressure is fine when the engine is cold, although the idle is still a little rough when the engine is cold. now once i hit the gas the oil pressure returns to normal and the truck runs fine.

i have noticed a few things...milky oil under the cap, no smoke from tail pipe, it takes a long time for the engine to warm up too, like 10-15 minutes of driving and then if you turn the heat on it's not hot, just warm. but the engine does not overheat. i have had to add oil so it's burning somewhere ..

i was thinking head gasket. after doing research on the site i have replaced oil pressure switch and got no change.or is it intake mainfold timing cover? heard to check screen at the bottom oil pan may be clogged. but why do these symptoms only appear when engine is hot? thanks.. and i want to add that once the engine is hot and has been driven...it does not start as easily as a cold start.. thanks again