i normally like to stick with the oil that the truck has been using but i belive that oil may be too thick.. try swapping it for a thinner oil and see if the noise stops.. then drive it for a good 50 miles let it cool down completely and start it again to see if it still makes a noise on the start up.. it could be that you just have a bad oil pump.. my father had a truck that had similar problems we thought the oil was wrong but it was a bad oil pump and just wasn't getting oil to the top end.. had a nice knock in it for about 30 seconds.. finally replaced it and it went away.. so don't scratch that off your list of problems just yet..