Startup knock

Okay the problem I have is really annoying me. I know my silverados but this one is got me stumped. I tried finding info online but not going anywhere. Here is the problem. I have a 97 silverado with 178000 miles it has the 5-7 vorteck. The truck runs and drives like a champ. When I start the truck in the morning it knocks for about up to five minutes and then goes away. It has good oil pressure and the oil I use is 10w 30 valvoline. With ac delco filter. The oil is not low. I live in south dakota and its winter right now. Once the truck is warmed up I dont hear anything. But when its parked all day and I go to start it again it knocks again. What can it be. Is it piston slap or csk is there a way to fix it. Should I run 5 30 in it. Use a different brand of oil. I know its winter and I should use 5 30 but with 178000 miles that shouldnt matter. The motor is well broke in by then. And advice on this will help.