rkmr

I have aa 95 nissan truck that has never let me down until now. Started hearing what sounded like lifter noises coming from the top of the motor so I changed out one side about 3000 miles before the other side. It is a v-6. The lifter noise is still there and getting gradully worse. nothing really changed. sounds like the lifters are getting no oil at all and are collapsing. sound is much worse when it is cold and gets somewhat better when it is warm but is still noticable. truck has 156k on it but leaks no oil, water or anything else. It still has good power and is still a very usable truck. Is it possible that the oil supply is just not getting to the top of the heads. Seems really dry to the eye but stick a finger in the oil cap hole and there is some in there. maybe worn oil pump? thanks for the help