Motor Trouble

I have 1999 tahoe 4x4 5.7 with 152,000 miles on it. My question is I've been having a problem with a sticking lifter. It does'nt happen all the time. I always keep my Oil changed. I use penzoil 10w30 In I always use a Quart of Slick 50 my question is the Slick 50 the problem or just high mileage.My last Oil change I didn't get slick 50 put in. Should I try a Oil flush or is this something that is more serious. It seems to happen first start up but after a couple of minutes it goes away. Could it be a weak Oil pump. Does anybody have any good suggestions. Thanks:banghead: